OMouse wrote:
Then there's the favourite of a lot of people, GTK. It's a little difficult to use though. It's been used for GIMP and FreeCiv.
Ad many many other things. Check out any *nix with a Gnome desktop for instance. It's pretty much all GTK apps.
Further, GTK needn't be difficult to use. GTK# in combination with Mono makes it quite easy to rapidly build GUI apps.
Also popular is using a language like Ruby or Python to write GTK apps. Python especially is being used all over the place now for this purpose.