So I did try mathoverflow a bit and it is a cool site. Over the few days I spent there I gained 593 reputation points, and no less than 9 bronze badges. The first answer I proposed gave me a badge as “teacher”, and the first question I asked gave me a badge as “student”. In fact, if I will only reveil my age I would gain also a badge as “autobiographer”.
Indeed, mathoverflow is ran by an energetic and impressive group of very (very very) young people (more precisely, very very young men). Here is the link to the 35 users with highest reputations.
Update: Well, I got a little addicted and visited quite a bit this nice site. Among other things, I tried to promote my fundamental examples question. Basic examples can give a quick invitation to wide areas of mathematics. (Maybe to most areas, I find it hard to think about a counterexample.) Overall, there are not that many basic examples and I think there will be a consensus about most of them so lists of different mathematicians will not be so different. More examples of important examples are welcome.
In my profile you can find there the six questions that I asked and the 10 answers or so that I offered (to other questions).
There is no definite answer yet to my question about typical behavior of random noise. (Update: Greg Kuberberg gave a convincing answer.) I still hope for an answer here or there. But I did found a new home to (and additional information on) functions of intermediate growth, like the function that satisfies the relation that were discussed two years ago on the “Shtetl Optimized”.
If you like discrete geometry you may be interested in this speculative question related to the Gallai-Sylvester theorem. (Update: some answers have led to a formulation of clear open problems and to some useful insight.) Overall, there are not many items in mathoverflow so far related to discrete/concrete mathematics. If you wonder what categorification is, look here for a few answers.
If you have an example of a mathematical example that has shaped a whole area of mathematics you can contribute it here. And if you want to tell a mathematical joke, or read quite a few, you can look at the question “do good math joke exist?”