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
• sci.stat.math
• sci.math.num-analysis
• 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.