Sidequest

""For a bunch of guys on a mission to save the world, you sure do love your detours.""

- Gig, Soul Nomad and The World Eaters

A very fundamental video game trope, a sidequest (or optional quest, or side mission, etc.) is any part of a video game that is not required to complete the game. Sidequests come in a variety of forms, and completing sidequests generally brings reward to the player such as additional equipment or abilities, areas to explore, supplemental plot related details, or fun unlockables.

Going out of your way and completing all sidequests results in one hundred percent completion.

Some side quests such as the Bonus Dungeon and Bonus Boss may provide challenges more difficult than any content available through the main storyline. This allows more casual players to still complete the game and see the plot resolved, while also giving gamers seeking an additional challenge something to go after.

Tends to be irrelevant to your main quest. Also see Infinity+1 Sword, Easter Egg, Fetch Quest, Quest Giver, Sidequest Sidestory. And That One Sidequest, though you probably don't want to.

Important to note that this trope appears in practically every game which gives the player even a slight amount of free rein, and is one of the best way for a developer to add more content and extend the length of a game.

Action Adventure

 * Featured in every single The Legend of Zelda game since the beginning of the series. A well-known example is Majora's Mask for its sheer number of sidequests, which translates in several characters in need of help, more complex mini-games to play and more secret zones to explore. Other games in the series, such as The Wind Waker, The Minish Cap and Spirit Tracks follow a similar trend. In general terms, the availability of sidequests in a Zelda game is inversely proportional on how many dungeons exist.
 * In Okami, sidequests help Amaterasu to gain Praise units, which gradually enhance her health, paint storage, money and revival chance stats. Some sidequests also house Stray Beads, a bonus supply of Gold Dust (which will permanently power up one weapon) and even secret brush techniques.
 * Solatorobo has a plethora of sidequests. They're optional, of course, unless you happen to need to be a slightly higher Hunter rank to take a plot-relevant quest. Not all quests will increase your rank, however.

Platform Game

 * Even the Mega Man games are not immune. Collecting plates in the Classic series; the armors from Mega Man X, the Cyber Elves from Mega Man Zero; and the RPG games Mega Man Battle Network and Mega Man Legends haven't even been mentioned yet...

Real Time Strategy

 * Most Command & Conquer games features optional objectives in missions, but Tiberian Sun went one step beyond and featured optional missions, unnecessary to progress further in the campaign but granting some sort of advantage in the associated main mission.

Roguelike

 * Nethack has two:
 * The Gnomish Mines, a Bonus Dungeon which has Minetown halfway down (with guaranteed shops and a temple) and a guaranteed at the bottom.
 * The Sokoban Bonus Dungeon, four levels with lots of food, a guaranteed ring and wand on each level, and either or  at the end of the final level.

Role Playing Game

 * Final Fantasy X-2 is a game constructed almost entirely out of sidequests.
 * Xenosaga Episode 2 was condemned for having what many have argued the vast majority of its gameplay be in the form of crappy Fed Ex sidequests and minigames.
 * The Elder Scrolls IV: Oblivion has many side quests, several of length and depth to rival the main plotline. Morrowind, its prequel, had much more sidequests, but nowhere near as many as Daggerfall, predecessor to both games, which was 99.9% sidequesting and a bigger game in general.
 * Though Daggerfall was filled with randomly generated side quests in a (more-or-less) randomly generated game world, while the later games had far more detailed, manually created side quests.
 * Skyrim and its Radiant Quest system treads on a lot of the same ground as Daggerfall's procedural generation to the same effect.
 * Baldur's Gate
 * Baldurs Gate is just full of lazy, lazy gits always asking you to go and fetch them a book, a sword, a dead body, a scroll, or something else that's often less than thirty feet away. To the point where your character has the opportunity to go on a long tirade that anyone who has ever played RPGs will agree with. Your journal will be full of snark about it.
 * Baldurs Gate 2 is famous (amongst other things) for having no pure Fetch Quests. Subverted and parodied in "Throne of Bhaal", where you subcontract a recovery quest to younger adventurers you just depetrified. They try to kill you for more loot, but reload the game after you slaughter them.
 * It also has one pure Fetch Quest but you have to go out of your way and wish for "A quest unlike any other(Sic)". You then have to find a gong which ends up being a cow dung shovel.
 * Planescape: Torment has a clever subversion of optional Fetch Quests. An old witch will only teach the PC magic if he fetches her three seemingly-innocuous and useless items: some herbs, rags and a fish. Not only do the apparently pointless errands actually have meaning attached to them but the items themselves are used to create spells.
 * Paper Mario had a good deal of them as well. The second game made it easier to find the minor ones by means of a Trouble center, where NPCs would put up help requests.
 * It is traditional in Shadow Hearts for every playable character to unlock a personal sidequest once The Very Definitely Final Dungeon shows up on the world map. These sidequests contain the character's Infinity+1 Sword, and usually fill out their Character Development. In addition, the later two games come with game-long sidequests for certain characters that complement or replace the last-dungeon-cued one.
 * This idea has unfortunately spread to the PSP game Crisis Core. There are 300 missions and just the very first one is required to continue with your game. What's worse is that there are only eight or nine "dungeons" the missions take place in with varying parts of them blocked off and all of them boil down to "find all enemies visible on the map and kill them", meaning the gameplay pretty much requires you do to the same thing over and over and OVER again...
 * The Last Remnant has a wide variety of sidequests, which is the main way to unlock map locations and the ability to hire some powerful people. There's also Guild Tasks which are similar in function but aren't classed as quests.
 * Chrono Trigger has six optional sidequests that can be performed between the end of the main quest up to the final boss and the actual fight with that boss. So much Level Grinding is needed to defeat it, though, that they are pretty necessary on your first playthrough, anyway.
 * Final Fantasy
 * Final Fantasy XII, has many sidequests, one example of which is killing of "marks".
 * Similarly, Final Fantasy XIII has sixty-four side quests available in Chapter 11. All of them revolve around killing a fairly powerful enemy.
 * The Dragon Quest series has quite a few of them, but the one that stands out is Dragon Quest IX, which has one hundred and twenty of them... that came in the box. With DLC, this gets upgraded to over one hundred and eighty. True, the majority of them are either Fetch Quests or killing a certain enemy a number of times and/or in a certain way, but the rewards are almost always worth it, such as class-exclusive armour, rare alchemy ingredients, or even new Vocations. This is a Justified Trope as well, your PC is part of a race of Winged Humanoids called Celestrians, who all have justified Chronic Hero Syndrome.
 * In Mario & Luigi: Superstar Saga, the Castle Town has two sidequests; you need to find a man's missing 'Beanlets' and dig up ancient artifacts for another man.
 * Neverwinter Nights. Hey, this Masterson guy in the Docks wants me to find his amulet (and I really wish they'd bothered to mark him on the map). Oh wait, my henchman wants this silver ring I found somewhere. Hmm, I wonder if the other henchmen want anything, like maybe this leaven bread recipe or this weird little brooch. The Many-Starred Cloak people want me to do all this magic stuff for membership and discounts. Oh, I can get money and quest XP for helping with the Tomb of Halueth Never thing. Oh, random if insultingly simplistic escort missions through areas I've already cleared of zombies. Hold on, some random druid wants me to engage in a minor act of ecoterrorism, there could be some dough in it for me. Oh, and my mad rogue skills mean I've been employed to burgle these random nobles. Bear in mind that this is a partial summary of the first chapter of four.
 * Mass Effect 2 has a subversion. The loyalty missions are under the plot-relevant "Missions", but are mostly kinda-sorta optional. You had better get One Hundred Percent Completion of them if you know what's good for you.
 * Skies of Arcadia has a ton, and you wouldn't realize half of them actually are sidequests. How can you tell? In Legends, there are special ranks for completing these things, included catching 1000 fish.
 * Exit Fate. The majority of your 75-person crew roster are optional. How do you get them? Side quests! And if you get them all, you unlock the Bonus Bosses. More side quests!
 * Dragon Age: Origins not only has a ton of side-quests, it even gives you an Achievement for completing 75% of them. The achievement is called "Easily Sidetracked".
 * Might and Magic tended to have a fair number of sidequests. VI, in a minor twist, made a fair number of them connected to the main story... in ways that you don't find out until the end of the sidequest, and maybe not even then if you miss a single chest or fail to read that letter you found.

Simulation Game

 * Rune Factory 2 has an optional sidequest composed almost entirely of fetch quests. The townspeople post requests on a message board that you fulfill to win their money and affection. Marvelous and/or Neverland apparently thought that wasn't enough, so Rune Factory 3 adds a mailbox and a message-delivering owl in addition to the message board. (Each one can only have one request fulfilled per day, so a total of three can be done per day if each one has at least one request.)

Turn Based Strategy

 * Final Fantasy Tactics Advance and its sequel both have tons of sidequests (out of 300 missions, around 30 are mandatory to see the ending). The sequel has up to 400 missions if you count random encounters etc.

Wide Open Sandbox

 * Pretty much every Wide Open Sandbox has side quests.
 * The Grand Theft Auto games all feature numerous side-missions that are not essential for completion, but often give you abilities that will make the game easier, such as fireproofing (for completing the firefighting mission) or the ability to get out of jail for free (for completing the vigilante mission).
 * The Simpsons Hit and Run has collector cards that contain items from previous episodes (Such as crab juice), collecting all in a level unlocked a multi player bonus track that was specific to the level, collecting all in the game allowed You (in level 3) trade all of them (although there still there in the pause menu) for a ticket to the "Itchy and Scratchy: 300 Yard Gash" from the Comic Book Guy.

Mythology

 * Herakles makes this trope Older Than Feudalism as he frequently had unrelated adventures (like wrestling Death to return someone that had died) while performing his 12 Labours. To the Ancient Greek tropers this was known as a Parerga

Web Comics

 * Adventurers!: Karn is obsessed with these, defending them with "Sidequests are an efficient way to increase experience", and once admonishing Ardam that he needs to get his priorities straight when he complains about the team going on sidequests instead of saving the world.
 * Roy Greenhilt, on the other hand, hates wasting time on sidequests.

Web Original
"Dagoth Ur: What the hell are you doing here? I'm the end boss! Ren: Correction, you were the end boss. That's my job now... Dagoth Ur: But you can't just skip to the end of the game without doing any of the 400 side missions!? Ren: Fine then, you can go tramping around this God forsaken rock for years on end, doing odd jobs for complete strangers, which in some twisted way ends up saving the whole world for no reason at all..."
 * Lampshaded in Ian's Adventures in Morrowind (archived here).


 * Deconstructed beautifully in this.