GTK+ is a GUI toolkit, originally designed for Unix and the X Window System.
This excellent toolkit has been ported to Win32 by Tor Lillqvist.
The port is based on the development version of GTK+.
Tor Lillqvist didn't only ported GTK+, but also The Gimp, a powerful image editing program (at least as good as Photoshop).
Go to his site to find out more about his ports.
Documentation
The API reference, tuturial etc. can all be found at the GTK+ website: http://www.gtk.org/
Download
Version 20000416, released at April the 16th, 2000