The Gimp Toolkit (almost always called GTK), grew out of the graphics program TheGimp?, to become a complete widget toolkit, eventually becoming the basis for projects such as GNOME and XFCE?. It was the toolkit of choice for a GUI programmer because Trolltech's Qt wasn't FreeSoftware, and the GTK was released under the LGPL.

It is written in C, although next came a project called gtkmm (for gtk--) that lets you use C++, and there are now bindings available in many other languages.

You'll recognise it. Look at some screenshots. Anything serious on Linux (Mozilla, OpenOffice and XimianEvolution to name some obvious examples) tends to use GTK.

The new 2.x series is fully internationalized using Pango and supports accessibility through the ATK libraries.

It has also been ported to Windows: see some screenshots.

GTK's website it at http://www.gtk.org/.

There's now also a version that uses Curses instead of X in development, allowing GUI applications to run on the console. It's called Cursed GTK and looks like this:

Reminds you of Borland's !TurboVision? IDE for DOS doesn't it? :)