Best free collaboration tool now that Google Wave is no more?

Question

With the unfortunate demise of Google Wave what is the best alternative free project collaboration tool that you use for your side projects and non-funded software start-ups?

Answer 1

So far this is my first choice:

http://www.teamlab.com/

Answer 2

Sign up with Runby, which integrates a Google Wave approach (hosted conversation) with email:

http://runby.me