Most researchers I know in my department make regular use of Stack Overflow, the question and answer site for programmers. While it is the oldest and most popular, there are a family of Q&A sites that closely follow the look and feel of Stack Overflow. More than a dozen of these target scientific and mathematical topics, and some are particularly relevant to those of us working in meteorology:

Mathematics

Discussions are more rigorous than the mathematics I use in my own work, but this site has helped me with PDEs and calculus, geometric problems, and numerical analyses

Physics

Perhaps the best place to learn about fluid dynamics, although I haven't used the site very much myself

Cross Validated

I have yet to use this site, but it's the place for questions about statistics

Geographic Information Systems

Again, I've not used this site except to get inspiration for shading relief maps and discuss the merits of rainbow colour scales

These sites are already well-established, and new questions are often answered within minutes. But there are some other sites that are still in beta, with Earth Science and Computational Science being the most relevant to meteorologists. These sites have smaller communities and new questions can go unanswered for days or longer. Answering a question on Stack Overflow often feels like trying to snipe on an eBay auction, but there is rarely such a race for upvotes on smaller sites.

Compared to Stack Overflow, I've found it easier to participate in the Earth Science community. As well as answering questions at a more leisurely pace, there are more opportunities to post new questions, too. And I think that my questions and answers tend to gain more attention at Earth Science than those on Stack Overflow.

The Earth Science site is still maturing and needs to attract more scientists to make it credible. At present there are, in my opinion, a disproportionate number of hypothetical questions and questions that are mostly for fun. There has been debate amongst the community about what types of questions are desirable on the site.

There are many other Stack Exchange sites that are not yet at the public beta stage. These are hosted in ‘Area 51’ and include several with topics relating to sciences, mathematics and teaching. Finally, the Academia Stack Exchange will explain how academics, students, and journals operate.

You can find my profile on Stack Overflow and Stack Exchange.