I’ve been a avid user of stackoverflow.com since I first heard of it a year or so ago. For the uninitialted it’s a place to ask development and programming related questions. The commuity is good and the idea of offering rep and badges to help others is great. Rep aint a new concept by any means but the way the stackoverflow team have implemented this simple idea is brill.
The new(ish) kid on the block is serverfault.com, which is aimed at sysadmins. A cool thing is you can link and import you profile from stackoverflow.
Also lats time I was on there i noticed the flair widget which allows you to embed your profile pic/rep/badge in any webpage.