Hm.. looks like a wiki of some kind. Might be self made.
Looks horrible to be honest..
/edit
reading
http://stackoverflow.com/about seems to suggest it is self made, then again, they could just mean the content not the backend.
/edit 2
http://meta.stackoverflow.com/ ; Welcome to Q&A about the
Stack Exchange engine powering these sites -- check out the FAQ!
/edit 3
http://meta.stackoverflow.com/questi...re-have-a-name
http://meta.stackoverflow.com/questi...etworking-tool
http://meta.stackoverflow.com/questi...to-other-sites
So, guess it is something they created themselves. I'm sure there's a package out there that does the same. I'm curios (sp?) what you've in mind tho.