Steam has its own category for indie games. Here’s a recent shot that lists the top selling indie games on Steam right now.
Quite interesting that Garry’s Mod (been there for ages) is selling so well taken into account that the game mod was published on 2006 (really cool!). It’s also quite interesting that World of Goo (80-90% of players are pirates) is also in the top. I’d wish to see more indie games getting there.