The GTK+ port to Win32 seems to me pretty nice, but I couldn't get it working. It's a port of the development branch, for one, which is unstable and under heavy development. I couldn't even get the GLIB header to compile under Cygwin, and when I fiddled with the #ifdefs that were causing it to fail, I ended up getting a few thousand compile errors instead of one. :P
The port itself requires no POSIX emulation layer (eg cygwin) and is written in straight up Win32 calls. It also includes extra goodies like a Win32 implentation of libpthread. The author of this port has also ported GIMP, because, well, that's why he ported the toolkit: to get GIMP working.
you can get it at http://user.sgic.fi/~tml/gimp/win32/