If you are a developer like me, you'll constantly be coming against new problems. Maybe there's a bug in an API you're using, or perhaps the documentation for a system just doesn't make it clear how to achieve a certain effect. If a google search doesn't suffice, then I tend to use an excellent site I stumbled across a while back.
It's a collaboratively-edited programming question and answer site. You post a question (like 'How do I get the focussed element on a page using JQuery?', and people post answers. On the surface it appears just like yet another forum, but I reckon it's much better. The key thing the designers added is a points system - every user has a rating that is shown next to every question, comment, or answer. If you ask a question deemed sensible or useful to other developers - you get points. If you post an answer, and it's marked as the best answer to the question, you get more points.
This feature obviously appeals to the competitive / slightly obsessive compulsive among us - people literally race to answer your question as quickly as possible!
For example, my slightly thorny iPhone UITableView question was answered correctly in three minutes!
There's an equivalent to stackoverflow for server admins too http://serverfault.com. It doesn't seem quite so active as stackoverflow, but it's still a great resource.