in reply to MathMonks?
I have not seen any web logs/forums/communities specialized for mathematics outside of some K-12 sites. I think such a site could be fun and informative, if it was not buried in homework questions.
There are active forums on Usenet:
- alt.math.recreational (occasional fun posts + much spam)
- sci.math.research (moderated)
These are not nearly as nice as Perlmonks, but interesting topics get discussed and people can sometimes get answers to questions.