List of Debian archive sites

http://ftp.XX.debian.org/debian/ stable (lenny) stable (lenny) release
http://ftp.XX.debian.org/debian/ testing (squeeze) testing (squeeze) release
http://ftp.XX.debian.org/debian/ unstable (sid) unstable (sid) release
http://ftp.XX.debian.org/debian/ experimental experimental pre-release (optional, only for developer)
http://ftp.XX.debian.org/debian/ stable-proposed-updates Updates for the next stable point release (optional)
http://security.debian.org/ stable/updates security updates for stable release (important)
http://security.debian.org/ testing/updates security updates for testing release (important)
http://volatile.debian.org/debian-volatile/ volatile compatible updates for spam filter, IM clients, etc.
http://volatile.debian.org/debian-volatile/ volatile-sloppy non-compatible updates for spam filter, IM clients, etc.
http://backports.debian.org/debian-backports lenny-backports main newer backported packages for lenny (non-official, optional)