I'm a regular user of the programming Q&A site stackoverflow.com and its companion for sysadmins, serverfault.com. Both sites have saved me a lot of time digging around the wilds of the web for answers, so I've tried to give back by answering questions whenever I can.
There are a few reasons why I like these sites: the flat organization of the questions and answers makes it really easy to narrow down the things you're looking for; the email and RSS notifications of new questions and answers are highly configurable; and - an unexpectedly good incentive for me - you can build up reputation points for asking good questions and giving good answers (as voted on by the other users of the sites).
There's now a similar Q&A site for software testers at http://testing.stackexchange.com - if Server Fault is Stack Overflow with a beard, ponytail and sandals, Testing.Stackexchange might be SO with half moon glasses and a clipboard. I think this could be a great forum to concentrate testing information; there's a lot of test related stuff on Stack Overflow already, but it's often buried with more development-oriented content. I'll be hanging out there regularly from now on.