I have of late, but how I know not, become rather addicted to stackoverflow.com:

It's currently only in private beta, open to about 2000 users, but I can see it taking off when it goes fully live.

It fundamentally similar to LinkedIn's Answers (but geared to programming questions) or expertsexchange.com (you can tell they really didn't think about that name beforehand - they finally added a hyphen recently) but free. Basically users ask questions and other users post answers. Users can also vote up or down other users' responses. How much you get voted up or down earns you reputation.

I have to admit to already having an addiction to the site. It seems genuinely useful in a way that none of the other Q & A sites are.
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