Realm of the Mad God is an intense bullet hell RPG that applies a permadeath mechanic, making the path to riches trecherous. It's addicting, but cruel.

It's a great co-op game, and if you have a lot of time on your hands it also really good for that too.

Order of Order endorsed: play it or something.

It must be good when people play the music for it in college while studying.

Crash cars with realistic physics simulation.

Great space sandbox. Create ships and bases with others; destroy or crash them with physics based damage, or just mine out asteroids and build for fun.

Find a friend and go to war.

Fun spy-like puzzle game with a unique mechanic.