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.
Update: (Sept 2010)A math site for university level mathematics exists now.
There are also sites for theoretical computer science and for statistics.
(April 2011) There is a TEX Q/A site!
(September 2011) Area 51: science contains new proposed stackexchange sites related to science. We can mention especially proposals for Theoretical physics (and mathematical physics) (closed), economics (closed), research economics, game theory, computational sciences, philosophy (the site is running), numerical models and simulations, Mathematica, mathematics in german.
