The GNOME software map contains a catalogue of software that is built using GNOME and GTK+. The catalogue is searchable and browsable, so provides easy access.
Some of the best known projects utilising GTK+ are:
GIMP (http://www.gimp.org/), an image manipulation program
AbiWord (http://www.abiword.com/), a professional word processor
Dia (http://www.lysator.liu.se/~alla/dia/dia.html), a diagram creation program
GnuCash (http://www.gnucash.org/), a personal finance manager
Gnumeric (http://www.gnome.org/projects/gnumeric/), the GNOME spreadsheet application
Glade (http://glade.gnome.org/), a GTK+ based RAD tool which produces GTK+ and GNOME applications
Anjuta (http://anjuta.sourceforge.net/), a versatile Integrated Development Environment (IDE) using C and C++ for GTK+ and GIMP.