SuperTux

SuperTux is a free and open source Linux based Super Mario Bros clone. It is inspired by the first and third games in Mario Bros series and even has a level editor and fan created levels that can be played to extend the game.

Like SuperTuxKart, this game is often packaged with Linux distributions and is available for many other systems as well, including Mac OS and Microsoft Windows.

(as of versions 0.5.1, 0.6.0 and 0.6.1)
 * Action Bomb: There are different types of enemies which explode when hit, such as Mr. Bomb, Haywire and Skydive. Short fuse is special as its explosion does not deal damage but instead knocks the player back, potentially into deadly pits or obstacles.
 * Artificial Stupidity: The most basic enemies will walk off platforms and into spikes and lava, causing their own demise.
 * Awesome But Impractical: Being Big Tux is often a good thing, as this gives you a second hitpoint. However, it's not so good if you have to make wide and precise jumps between small platforms. In levels requiring this you are better off as Small Tux.
 * The air flower, if you collect a single one. It lets you move faster, which is not always a good thing. And it lets you jump a bit higher and hover briefly as you fall. Until you get used to the timing, this can cause players to miss platforms they would have hit without the powerup. However, with multiple air flowers things get more awesome, as you can hover for a longer time.
 * Big Bad: Nolok has captured Tux's girlfriend Penny and forces him on a long and dangerous adventure to get her back.
 * Brutal Bonus Level: There are many bonus levels which range from very easy to freaking hard. (And with help of the level editor, the users can also create their own levels, which will be kind-of bonus levels in the game.)
 * Incubator Island (actually a set of incompletely implemented levels thought for testing) has a few very hard levels that require many nigh-impossible jumps and dodging of a bunch of Goddamned Bats and Demonic Spiders. These levels tend to cluster in the lower right of the island.
 * Convection Schmocvection: In some levels Tux can stand on platforms less than one tile above a lava surface without getting hurt. Also when he has a powerup and falls into a non-bottomless Lava Pit, he will not die instantly but instead take a hit, giving the player a chance to get out alive (if the pit is shallow enough; otherwise Tux will subsequently die).
 * Cranium Ride: Ice crushers and their Forest World reskins.
 * Crippling Overspecialization: The earth flower is really useful in dark underground levels, as it gives Tux a "safety helmet" equipped with a little lamp. Obviously, that lamp is effectless in a lighted environment. Although the earth flower still has an effect - it gives you an extra hitpoint (like the other flower powerups), and allows you to "petrify" briefly and become invulnerable. Hitting the key combination for triggering the petrification at the right moment is difficult, however.
 * Tux in general has shades of this in current versions, being a penguin who can't swim.
 * Damsel in Distress: Penny.
 * Death Throws: Tux does it whenever he dies, while flailing his wings.
 * Happens to some enemies, too (depending on the way they die). Since version 0.6.0 the Yeti boss does it as well.
 * Depth Perplexion: Played with in some bonus levels and fan-made troll levels.
 * Difficult but Awesome: A single ice flower. It lets you throw ice balls that can freeze many enemies in place and extinguish fire, but only one at a time. And the iceballs tend to ricochet for several seconds before disappearing. Meaning if you miss your target you have to wait until the iceball has disappeared (or hits by chance after bouncing off several obstacles) until you can throw the next. However, with some skill you can hit enemies indirectly using obstacles as "reflectors". This is not possible with a fire flower.
 * The in-game level editor.
 * Everything's Better with Penguins: Tux and Penny.
 * Family-Unfriendly Death: While not overtly shocking, the death animations when mooks are hit by fireballs are a bit more graphic than those in a typical Super Mario platformer. Susceptible ice and snow enemies will melt while those of Forest World will burst into flames briefly before disappearing.
 * Flunky Boss: Ghost Tree is surrounded by special will-o-wisps that you have to use in a particular way to defeat him. But don't touch them or you get hurt.
 * Goomba Stomp: Many mooks can be defeated this way. Those who can't usually either have obvious spikes on their top, or they are ghost-like entities. Also the first boss has to be defeated by repeated jumping on his head.
 * Tux can also springboard from squished enemies. This is even required in a few locations, and can be (ab)used to bypass parts of a level in others.
 * Ground Pound: Big Tux can "butt-jump" and break wood blocks under him. This move also kills frozen enemies (with a few exceptions).
 * The Yeti boss uses it as his main attack pattern... indirectly.
 * Hard Levels, Easy Bosses: The developers avoid making bosses too easy, but SuperTux still counts as this. For example, look at the sheer number of levels on Icy Island that you have to go through in order to reach the Yeti boss, and then look at the boss fight itself. Which one will likely take more time?
 * Hoist By His Own Petard: Some enemies can be stunned and then kicked around like Mario's koopas. Like these, they can defeat other enemies when sliding around, but they can also hurt the player himself when they bounce back from an obstacle.
 * Also, there are particular blocks which burn away when hit by a fireball. Be careful when walking over these blocks with a fire flower, especially when there is a Bottomless Pit underneath.
 * Mooks hit and frozen by an ice ball mid-air will fall down and hurt Tux when he is below.
 * Made of Iron: Tux, as long as he has enough coins to lose when he dies. At the same time, he is a One-Hit-Point Wonder when he has no powerups.
 * Played straight when Tux has caught an invincibility star.
 * The Yeti boss is insensitive to fire or ice balls, unscathed by his own stalactites, and he can walk over spikes and through lava if necessary.
 * Obstructive Foreground: High grass in some Forest levels hides small mooks nearly completely, so that you walk into them if you're not careful. Also there are areas of darkness in some (other) levels where the view is obstructed by a black foreground.
 * 1-Up: The Tux puppets are kind-of this. Each one grants the player 100 extra coins if they catch it. And coins are needed to restart from the last checkpoint when Tux dies.
 * Playing With Fire: The Fire Flower, analogous to that in Super Mario Bros.
 * Schizophrenic Difficulty: Given the relatively large number of levels per world, SuperTux inevitably has shades of this. (In part also because both implemented worlds feature different environments throughout their courses, such as Underground Levels, sky levels, water-rich levels and castles.) This architecture makes it very hard to constantly ramp up difficulty.
 * Slippy-Slidey Ice World: Icy Island, the start point of the game. Usually Tux has no problems walking over snow and ice tiles, but there are surfaces which are so slippery that even the little penguin will slip (oddly enough, this doesn't affect enemies).
 * Super Drowning Skills: Ironically, Tux has them in current versions, because swimming is not yet implemented. So he just falls through water as if it was air (unless one uses tricks like wind and low gravity).
 * The Goomba: Snowballs on Icy Island, and poison ivies in Forest World.
 * The Lost Woods: Forest World, the second world after Icy Island. Increasingly haunted in later levels.
 * The Unfought: In current versions, it appears as if is this. However, one day he will be added into the game... probably. For those who can't wait, there are already a few fan-made boss fights where you can combat him directly.
 * Waddling Head: Many mooks are this.
 * When All Else Fails Go Right: Works in most levels.
 * When Trees Attack: Walking tree (a mook) and the Ghost Tree boss.