Here is a brief list of science-related sites that run on the same platform as Math Overflow:
- Science
- Physics
- Electronics
- Programming
A complete list of these sites is available here.
The one thing that I personally feel is missing is a lower-level mathematics site (a Math Underflow, if you will). I understand the desire to keep Math Overflow relevant and interesting for professional mathematicians, but there is currently no equivalent site for amateurs and students.