Stack Overflow

Awhile back a friend and co-worker showed my Stack Overflow. I never really got into at the time, but lately I have had need to ask some questions. Mostly, I really love the way they have organized the workflow and ratings of question/answer. It goes way beyond the email list thread idea. On top of that it is something of a social site as well. Your user gains reputation from posts/answers and voting. I was amazed at how quick I got answers but more than that how thorough people are to provide a clear and concise answer. The reputation aspect really encourages others to answer well.

Also, they are a service which uses Gravatars (see previous post).