Defog of War

Fog of War is something that often annoys people, being unable to see enemy units and such in it, even worse if the computer is unaffected by it and can prey on your units with ease. Fortunately, there are tools at their disposal to clear the fog and see whatever was hidden inside.

Role-Playing Games

 * Pokémon Diamond and Pearl: The move "Defog" removes fog outside of battle and reduces target evasion among other things in battle.
 * In the Fog of War levels in the Metal Gear Acid games, the IR Goggles, Solid Eye, Major Zero and Marionette Owl cards will let you see easier, although each one has its own drawback (some let you see the entire area but only last a turn, some last forever but only increase your field of vision a little, some are good but cripple you with added COST for as long as you have them equipped, etc).
 * The Solid Eye in Metal Gear Solid 4 has an infrared mode that allows you to see through snowstorms quite well.
 * LEMMESI in Desktop Dungeons shows you three random squares on the map. Because removing Fog of War heals you, the spell is effectively free (it reveals enough squares to repay its MP cost) and lets you heal without having to leave your current battle.
 * Persona 4: there's fog in the TV world, but the protagonists wear glasses made by Teddie to help them see through it. The fog, however, plays no part in gameplay. (Until you get to the  fight. Oddly enough, the glasses do nothing to combat his Fog attack.)

Real Time Strategy

 * StarCraft and StarCraft II has the Comsat, which reveals a targeted area on the map.
 * Ironically, most professional players prefer not to use them this way since cloaked units are such a pain. They'd rather sacrifice the SCV rather than scan for information since the energy can create the much more effective mule.
 * Warcraft III has the spell "Farsight". Paladins had Holy Sight or something like that in II.
 * The game also had Goblin Workshops that allowed you to do the same thing as Farsight though for a price.
 * Plants vs. Zombies has Planterns, Blovers, and Torchwoods to remove fog in the night pool level.
 * Dune II: when you start, you can only see a limited area around your Construction Yard. Once you create an Outpost you get an on-screen radar which allows you to see the entire area you've explored.
 * There is one in the first Dawn of War at the disposal of the Imperial Guard called something like an 'auspex'. Basically an ability of the command center to reveal a circular section on the map, and show any stealthed units in said circular section. Of course, The Computer Is a Cheating Bastard, so it always has the ability active, reusing it once it ends, and always hits your stealthed units with it.
 * The first Red Alert goes as far as give the trickier faction GPS satellites, which permanently reveal the battlefield, and gap generators, which generate it for the enemy. The Russian faction received spy planes, which are much slower in revealing the map. Normally fog of war stays gone after it's dispelled.
 * This set-up remains for Red Alert 2, though if one's radar facility is destroyed or sabotaged by spies the fog is reset. In Red Alert 3, Allied commanders have a recon sweep power which unveils everything from point A to point B - terrain is always visible to all commanders, though.
 * Total Annihilation and Supreme Commander do not have defogging, but do have radar, which shows units in the fog on the minimap. Large battlefields and ridiculous artillery make this vital.
 * EndWar has the ECM support; it stuns enemy vehicles but it also shows any units out of Line of Sight or any Stealthed Riflemen.
 * World in Conflict had the "Aerial Recon" Tactical Aid, which revealed all enemy units (including the hidden ones) in a certain area. Useful to finding those pesky enemy artillery units, as well as unmasking the infantry hidden in the woods and the tanks hidden behind smoke shields. The latter can also be done with the special ability of the Scout Helicopters, which basically does the same but limited to the scout's current location.
 * There is also a defogging ability in the Elves' arsenal in LotR: Battle for Middle-Earth, at least in the second one. It clears a blurry-edged circle on the map.
 * This option is avaiable in all of the Total War Series games.
 * The Clairvoyance Support Power, as well as Twisted Fate's Limit Break both incorporate this in League of Legends.
 * In Star Wars: Empire At War, every faction has one unit which has the ability to clear a defined area for a fraction of time via sensor ping. On certain maps you can also capture a radar station to remove the Fog of War permanently on the entire map.
 * In Age of Empires III, your explorer has a "spyglass" ability, which instantly reveals a circle in the fog of war, anywhere on the map. Early in the game, this can be used to scout out resources or find your opponent's settlements. Late in the game, it's mainly used to spy on your enemies and find out the make-up of their army before an attack.
 * It's also possible (very late in the game) to research the "Spies" upgrade to completely remove the fog of war. However, by the time you're able to afford researching "Spies," you've already mostly won.

Turn-Based Strategy

 * Fire Emblem: the Torch item and Torch Staff are useful in increasing vision in fog or night. While most units can only see a few spaces around them fog or night, the Thief, Assassin, and Rogue units can see a considerable amount of spaces around them, making them invaluable in such scenarios. Once shifted, Laguz units in the Tellius games also have a better sight than most of your units.
 * Interestingly, the Torch doesn't light up a fixed radius around its user - it adds to the user's sight by a fixed amount. So a torch held by a thief, assassin, or rogue is three times brighter than that same torch held by anyone else.
 * Advance Wars: Sonja's main strength is her increased vision range in fog.
 * The Flare unit in Days Of Ruin
 * The Recon unit in all four games (and probably previous Nintendo Wars games too). The Submarine, too.
 * Some races in Warlords Battlecry 2 can have an upgrade that exposes the whole map and removes the fog of war.
 * In Heroes of Might and Magic series, giant trees and mage huts with magical eye networks served precisely this purpose, the first one revealing a big area around the tree, the second - a moderate area around the eyes. The Grail building for the Academy, a flying ship, was an instant Defog of the entire map.
 * In Civilization, discovering satellites reveals the entire world map (and causes NPC civs to ask to trade world maps more often, which only gets you updated info on new/captured settlements at this point). Air units can preform a recon action, which does as you would expect and reveals a part of the world that is in-range.
 * Age of Wonders 2+ has capturable structure Watch Tower that gives a good view (and defensive walls) and Reflective Pool that shows some other area.
 * Battle Isle series had at least dedicated recon (as in, without attack or support capabilities) plane.
 * Advanced Strategic Command has Spy Satellites and various radar units, including stationary ones. And there are air-droppable sonar buoys, in case the enemy got submarines. When unavailable, a player may use some Anti-Air units and aircraft; makeshift scouts include snipers (fragile, harm only infantry and has view range only slightly above basic, but may get closer due to being stealthy) and submarines (those that can't attack land units still can see targets on a coast without exposing themselves) - anything with good view value and/or high stealth can give an advantage. Of course, it becomes especially important when using long-range weapons - few artillery and no units with cruising missile can see to their maximum attack range, and some of them have weapons with minimum attack range exceeding their view range, i.e. useless without some sort of a forward observer. And of course, it's important while trying to either use Hit and Run Tactics or counter it.
 * In Emperor of The Fading Suns there are a few otherwise weak scout units - mainly Scout Tank and Tracker Legion. Special Forces and Spies can do it as well, but are a lot more expensive, a little slower and with mean attacks.