[Sushi]

http://www.digital-eel.com/sais/

Free game, designed for short sessions and high replay value. If I had to compare it to anything, it would be Star Control II: but much smaller scale and a lot more random. You wander (a small part of) the galaxy, find artifacts and weapons, encounter aliens, fight some of them, try to make it back home alive and on-time. Each game takes 15 minutes or so, and each game is different from the last.

[BloodEagle]

The only problem with it is that it's a game of chance.