Extreme Programming

http://www.extremeprogramming.org/
Possibly the tightest, most focused site on XP. Don Wells uses a neat little XP process diagram to let you navigate through the life of a project. An extra bonus is that you can download the site in ZIP format for off-line viewing.
 
http://www.c2.com/cgi/wiki?ExtremeProgrammingRoadmap
The Wiki Wiki starting point for Extreme Programming. The site is a place where you can get involved in the ongoing dialogue surrounding XP.
 
http://www.xprogramming.com
A Web site run by Ron Jefferies (one the "fathers" of XP). The site also includes the on-line, XP Magazine, which is a great "warts and all" look into XP. Quite a bit of very good content on this site.
 
http://www.xp123.com/
William Wake's Web site (the author of Extreme Programming Explored). Lots of content here; including fine examples of refactoring and test-first programming in action. Bookmark this one!
 
http://groups.yahoo.com/group/extremeprogramming
Don't just sit there; join the XP group on Yahoo and enter the fray! Post questions, vent frustrations, enjoy a joke and get some answers

Agile Development

http://agilemanifesto.org
If you feel a surge of passion for Agile development; drop into this site and sign your name to the Agile manifesto. Failing that you could at the least print-out the manifesto and stick it the side of your cubical!
 
http://www.agilemodeling.com
Scott Ambler's Agile Modeling Web site. This has a fairly wide-range of essays and articles on AM and links to other related modeling sites. You'll also learn how you can use the AM logo on your Web site!

XP Development

http://www.junit.org
Get the latest JUnit test framework from this site. It also includes numerous links to other related XP sites and is still very useful even if you're not developing in Java. The testing approaches described on this site are pretty much transportable across to other non-Java languages.

http://www.nunit.org/
The Microsoft .NET framework version of JUnit.
 
http://Jakarta.apache.org/ant
The Web home for the Ant automated build tool. Check for the latest version of Ant at this site once and while. You will already be familiar with this site after your work with automated builds.