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