These are some sites I'm involved with
Extreme Programming
Extreme programming in Belgium: people who are interested in Extreme Programming in Belgium. There are regular meetings, a wiki site and a mailing list.
Extreme programming in The Netherlands: the Dutch XP users group. Many of the Dutch and Belgian XP users attend both groups and the groups sometimes hold joint meetings.
Conferences
XP Day Benelux conference: A one-day XP conference organized by the Dutch and Belgian XP/Agile users groups with Agile Systems vzw.
WIKI sites
Bookshelved: Laurent Bossavit's wiki discussing books.
Ward Cunningham's WIKI was the original wiki that started this whole thing. Spirited discussion about patterns, Extreme Programming, lots of stuff related to computers and much more!
Systems Thinking sites
Dutch-Belgian Systems Thinking wiki: Dutch, French and Belgian software development consultants hold regular workshops to examine real-world cases using 'Systems Thinking' techniques. You can participate in the Systems Thinking Wiki
Agile Systems vzw: Belgian non-profit organisation that gathers and disseminates information about 'Systems Thinking' and Agile techniques.