This is a list of notable applications that use GTK and/or Clutter for their GUI widgets.
Such applications blend well with desktop environments that are GTK-based as well, such as GNOME, Cinnamon, LXDE, MATE, Pantheon, Sugar, Xfce or ROX Desktop.
The GNOME Project, i.e. all the people involved with the development of the GNOME desktop environment, is the biggest contributor to GTK, and the GNOME Core Applications as well as the GNOME Games employ the newest GUI widgets from the cutting-edge version of GTK and demonstrates their capabilities.
Despite the immense popularity of Qt, there continues to be science software using the GUI widgets of version 2 of GTK toolkit.
Whether this is going to remain that way, or whether the software will be ported to some current version of GTK (maybe GTK 4) remains to be seen.