MathOverflow turns two years old this week — a milestone in the transformation of mathematical research into a massively collaborative endeavor. It’s happening on blogs, it’s happening on mailing lists, and it’s happening in a big way on MathOverlow, where mathematicians ask and answer the sorts of questions that might come up in the faculty lounge — if the faculty lounge were populated by hundreds of experts pooling their expertise.

If you’re interested in mathematics at the research level, MathOverflow is a place to learn something new and fascinating every single day. (If you are not doing mathematics at a research level, feel free to read but please don’t feel free to join the fray; questions at anything below about a second-year graduate level should be directed to MathStackExchange, another massively collaborative site aimed, roughly, at the college level — which reminds me that it’s not just mathematical research, but also mathematical education, that is being revolutionized before our eyes.)

It has been absolutely fascinating to watch MathOverflow and its sister sites develop. The mathematical content is awesome, but so is the sociological phenomenon — problems solved in hours instead of months via virtual brainstorming among some of the smartest and most knowledgable people in the world.

When MathOverflow first came on line, I thought it would be a superfluous addition to the many electronic resources already available. I couldn’t have been more wrong. I now suspect that like all other toddlers, it will be awesome at age five in ways that are only dimly imaginable at age two. If you like mathematics, these are very good times to live in.