Applications

The best GTK+ screenshots are from applications which are successfully using GTK+. Here are a few pages which show off such applications:

GIMP
GNOME
Glade

Ports

Other interesting screenshots can be seen at projects aimed at porting GTK+ to alternative display systems, instead of using the X Window System:

Linux-FB: testgtk testtext testtext Dia
BeOS DirectFB

Themes

The best place to see GTK+ theme screenshots is to just browse around art.gnome.org.