There is a CVS server on netfilter.org which contains the latest HOWTOs, userspace tools and testsuite. For casual browsing, you can use the Web Interface.
To grab the latest sources, you can do the following:
cvs -d :pserver:[email protected]:/cvspublic login
# cvs -d :pserver:[email protected]:/cvspublic co netfilter/userspace
cvs update -d -P