Art.gnome.org
Themes and backgrounds for use with the gnome desktop environment
http://art.gnome.org/
GTK 1.x Themes
GTK 1.x Themes
http://themes.freshmeat.net/browse/923/?topic_id=923
GTK 2.x Themes
GTK 2.x Themes
http://themes.freshmeat.net/browse/958/?topic_id=958
Sawfish (Gnome Window Manager)
Sawfish is a database that anyone can edit.
http://sawmill.sourceforge.net/
Sawfish Themes
Sawfish Themes
http://themes.freshmeat.net/browse/926/?topic_id=926