Fishy is a Flash game that's been floating around the web for quite some time. What's remarkable about it is that it is incredibly simple, yet nerve-wrackingly difficult to master.
The premise is so simple you could pick up the game without instructions. You, a fish, are dropped into a tank. Fish swim by—some big, some small, some HUGE, some tiny. Your job is to eat any fish that are smaller than you, which makes you grow. As you grow, you can therefore eat larger fish.
Controls are with the arrow keys, but beware that the game has a sense of inertia and resistance—it takes a second or two to accelerate to full speed, and you will slow down and stop without pressing any keys. Hitting the left or right edge of the tank will cause you to appear on the opposite edge.
Fish eat other fish merely by touching each other; it doesn't matter which direction a fish is facing, or which part of the other fish it touches. When two fish collide, the larger survives.
Touching ANY fish larger than you ends the game, period. No life bar, no extra lives, no continues, the game just ends. This, combined with the less-than-instant response of the inertial controls and the random nature of the fish that sweep by make the game difficult (and sometimes maddening). After playing it for about two years, I've still only managed to beat it, oh.. twice.
Xgenstudios, the authors of this game, recently updated the game, which now starts you at level 1 and allows you to progress through the levels as you eat fish. Each level features larger fish. In theory, this would make it easier to beat, but in practice, I keep losing.
Personally, I think I enjoy Fishy Classic a little better than the new version. Also, the music and sound effects are adorable (and despite being horrifically repetitive, actually pretty catchy).