Here are some of my favorites that are $15 and under:
*Matt Hazard: Blood Bath and Beyond
*Flower
*Zombie Apocalypse
*PAIN
There's more that I haven't tried, but I'd also reccomend Battlefield: 1943. It's $20 I think.
I'd extremely reccomend Flower though, it's a wonderful game.
Edit: There's also .detuned if you're the creative type.