The following Web sites are useful resources:

groups.yahoo.com/group/refactoring. A group for discussing refactoring.
groups.yahoo.com/group/extremeprogramming A group for discussing XP. There's a lot of discussion unrelated to refactoring, but refactoring is one of the key practices in XP.
www.refactoring.com. Martin Fowler's site, associated with Refactoring. He has most of that book's catalog online, along with contributions from others.
www.industriallogic.com. Joshua Kerievsky's corporate site. He's working on a book, Refactoring to Patterns, and has interesting articles and games as well.
www.xp123.com/rwb. In general, my site is focused on XP, and the rwb area is devoted to this book. You'll find source code for the larger examples, refactoring-related articles, and pointers to refactoring challenges others have proposed.