I started playing Atom Zombie Smasher. Once you get into it, it's surprisingly a fun addicting game.
Braid and P.B. Winterbottom was pretty cool too.
Can anyone recommend any other good ones?
Recommended in no particular order:
1) Realm of the Mad God (Free)
2) Limbo
3) Minecraft
4) Heroes of Newerth
5) Cave Story
6) Super Meat Bot
7) Machinarium
8) World of Goo
9) Frozen Synapse
10) P.B. Winterbottom
11) Braid
12) Atom Zombie Smasher