http://altitudegame.com/
I stumbled upon this game earlier whilst browsing around the aloha forums, and I must say it's definitely the most fun I've had in a while. The game is basically a 2d airplane shooter, and you can customize your plane with perks and stuff. Sounds pretty standard, but the simplicity and addictiveness makes it stand out. The best part is that the free version doesn't expire. It just limits the stuff you can buy at high levels, which takes way to long to get to, anyway. There are several gamemodes:
-Soccer, where you want to grab the ball and fly it into the enemy net. The catch is that you can shoot and destroy the enemy
-Deathmatch
-Team deathmatch
-Free for all
-Demolition
-One life (Assorted gamemodes, but you are eliminated after your first death)
Software requirements: NONE -- My crappy old computer loads the game up without lag in about 5 seconds.
Once again, you can find everything you need here: http://altitudegame.com/