There's lots of XP-related material, great patterns stuff, and many other discussions on Ward Cunningham's wiki. Start at http://c2.com, and find the XP material starting at http://c2.com/cgi/wiki?ExtremeProgrammingRoadmap.
extremeprogramming.orgDon Wells has a "gentle introduction" to XP, and lots of other good material on his site. Also check out Don's great XP links.
Object MentorObject Mentor is the home of the famous XP Immersion course, and a number of good friends and teachers.
www.testdriven.comA site promoting "techniques, tools, and general good will in the test-driven community". Jeff Miller, Eric Vautier, David Vydra.
Methods and ToolsFree e-newsletter providing practical knowledge for software development professionals.
XPdeveloperXPdeveloper is a UK-based group focused on learning XP by doing.
XP-FranceXP-France is the French association devoted to XP.
CAMUGCalgary Agile Methods Users Group
XP-Columbus, OHColumbus XP Group. Monthly meetings!
XP-JapanJapanese mailing list devoted to XP.
XP-SwedenSwedish XP wiki.
XP-SwissSwiss XP / Agile site. Articles mainly in French.
XP-UkraineThis site written in Russian, contains Extreme programming materials only. There are separate sections for brand-new pracitioners and professionals; theory pages; articles; software. In the future there will be books and XP-quiz sections.
Extreme Programming BrasilEXTREME PROGRAMMING BRASIL, primeiro congresso brasileiro de metodologias ᧥is de software.
XP-ChinaYou guessed it! China.
XP-Ireland NEWYep, Ireland. XP-oriented yahoo group.
XP-NetherlandsDutch XP user group, text largely in Dutch.
Extreme Programming Mailing ListObject Mentor started this list and keeps it ad-free. It's run, however, by an unruly mob of XP enthusiasts, students, and critics. Very active. Enter your email address and push the button.