Pittoch (Windows, 1.2MB, free) - A superb little puzzle game where you must gather keys to unlock the exit on each stage. Each blobby critter can hold one key, and in order to control extra ones you must link them together. Linked blobs move as a unit, so if you have two holding hands you can't fit through a single-spaced gap between blocks. Simple but highly entertaining and just the right amount of challenge.
CryptRover (Windows/Mac/Linux, 3MB, free) - A roguelike that puts you in the role of archaeologist trapped in an underground crypt. You have a limited air supply, which can be replenished by picking up canisters laying around the levels, and spiders are waiting to attack you at every turn. It plays less like a roguelike and more like an action game, but the concept combined with the ASCII graphics are great.
Big Building Boom Blues (Windows, 1.9MB, free) - After wiring a bomb at the top of a skyscraper, you must rappel down the side of what is probably the most strangely contoured building ever made. Can you reach the bottom before the explosion catches up with you? If you don't, the game calls you dumb...
Armand and the Foppish Hat (Mac, Windows, 20MB, free) - A short Zelda-like adventure where you help Armand on a quest to... save the world? No, not quite. Save a princess? Nah. Restore peace, honor and justice to the land? Nope. How about recovering his spiffy hat? Even better! Simple visuals but a nice sense of humor make this one a good afternoon diversion.