mod_rewrite forum -- The place to exchange ideas, code, and frustrations!
Beginner's Guide to Mod Rewrite - Start here.
Apache.org documentation -- The original documentation.
Ralf Engelschall's Rewriting Guide -- Numerous examples from the man who invented mod_rewrite.
Apacheref.com documentation -- Nice quick-reference.
Altaway documentation -- This seems to contain the best description of the various flags that can be used with mod_rewrite commands. Also contains plenty of general information about using .htaccess files.
search engine results -- Overwhelming, of course.