I just recently discovered this StackOverflow implementation focused on Agile issues: http://agileshout.com. Seems to be low traffic at the moment, but perhaps my agile friends can find some value there.
I'm not quite sure why it's necessary to have a separate site from StackOverflow, since the tagging mechanism in StackOverflow permits differentiation of topics (e.g. "agile") and cross tagging of topics (e.g. "agile" and "BI") that might not otherwise find a specific home on a specific site.