GTK 1.x & 2.x -- I believe these are programming libraries primarily developed for programs running under the Gnome Desktop
-- Window Manager for Gnome Desktop
-- 3-D desktop effects/window manager
-- Fork of Compiz because they wanted to do things a little different
Themes -- GDM is the graphical greeter (login screen) for Gnome desktops. KDE typically uses kdm. Generic X uses xdm.
-- Progress bars, etc during the loading of the OS.
Themes -- I believe XMMS is a music player.