there is also the gnome-art package available in the repositories (in Ubuntu anyway)
"Gnome Art is a tool for downloading and installing GNOME themes
from
http://art.gnome.org/ website. It provides a nice theme list
with options to preview, download and install them."
Also, if anyone ever wonders what the different theme types are (as I did at first) - GTK2, Metacity, etc.
this thread explains it all well.