What are the best free games on steam?

I'm curious because I never really go out of my way to look through free games on steam.. because generally I think they aren't very good. Even though there are a lot of bad priced games out there. However I'm sure there are many good free games on steam... At the moment I'm going to play a free game called PRICE (what an ironic name) because I happen to find it on the main store - it looks pretty good.

So what do you think of the free games available on steam?
Can you recommend any?

GA (Level 2+)