If you've searched Google for software development questions in the past few months you've probably found links to Stack Overflow, the question and answer community for programmers. Best charts in Python? Next/Prev links in Rails? jQuery array operations? Stack Overflow's got answers.
I've always liked community question sites. Ask Metafilter is my favourite, it's a good community and the questions are eclectic. Most other question sites have failed. Yahoo Answers is a cesspool, Experts Exchange is downright deceptive, and Google Answers with its paid researchers is sadly gone. Stack Overflow and its sister sites for sysadmins and users have a strong community of experts with a good attitude.
Two key features of Stack Overflow are Digg-style voting and Advogato-style reputation scores for users. Voting helps good answers rise to the top, often augmented by comments from other experts. And reputation serves as a compelling incentive to answer questions quickly. I'm surprised how often I check the site to see if there's a question I can answer and how quickly I jump to help someone if I think I'll get up votes and increased reputation. The reputation game does encourage some unfortunate behavior, but in general it seems to work.
The folks behind Stack Overflow are understated and quite modest. But they may be crafty; I think the site they're building is really valuable. There's already some modest advertising on the site, but the real value may well be in a jobs board. An active community of programmers, everyone annotated with their areas of expertise and ranked by helpfulness? That's gold.