The C++ Interface to Gtk+
Can be used to create nice looking GUI applications, all based in C/C++.
Serves as a foundation for the GNOME Project, but also can be used for stand-alone apps.
Is multi-platform, allowing your apps to be deployed on many different platforms: Linux, Windows and Mac OSX (coming in version 2.10)
Gtkmm (orginally known as gtk--) is a C++ wrapper, or language binding, for GTK+.
Supports C++ features such as inheritance, polymorphism and other powerful C++ features.
It offers a type-safe interface so you can catch GTK+ runtime errors at compile time.
You can use class types on the stack, allowing you to avoid having to deal with raw pointers (GTK+ works only with bare pointers).
Allows actual derivation of widgets easily, which apparently is very challenging in GTK+.
Uses libsigc++ to handle GTK+ signals, which are inherently not typesafe. GTK+ signals require functions or static methods. libsig++ allows connection to class methods.