Question and answer site for mathematics, with quite a bit of serious activity at a high level. - Michael Nielsen

It would be wonderful if this took off like Stack Overflow. - mkz

I see they're grappling with the perennial LaTeX problem. http://mathematics.stackexchange.com/questio... - Dan Hagon

One day we will have this for synthetic organic chemistry. One day soon. Mark my words. - Matthew Todd

mkz - I looked through about a dozen comment threads. Some of the content is too far from my area for me to judge, but it looks to me like they have a very nice little community of professional mathematicians contributing. Provided they're each getting more out of it than they're putting in, it's easy to see this gradually becoming more and more popular. - Michael Nielsen

It looks like some of the usual suspects of the math blogosphere are there. Hopefully they will hang on for a while, at least long enough to create enough high quality momentum to get the thing going. There are a few high quality email lists that serve similar purposes, but the level of discussion on the ones I'm subscribed to is maybe a bit too high; it seems a bit hard for non-senior people to ask their "stupid" questions. Math Overflow will hopefully serve a wider audience. I think the voting/reputation system served well for Stack Overflow, I'm curious to see how it will pan out with the math crowd. - mkz