GTK+ is used by a large variety of programs on a decent number of platforms. It doesn't have the best reputation for certain bits and pieces, some fairly, some unfairly.
Discussions have begun on what will be the new version of GTK+ 3.0. The most important news is that they will be able to break the ABI which they have heroically maintained all through the 2.0 releases, this should mean lots of good stuff now they are let off the leash.
You can check out the start at this blog post.
The touchstones for the GUI toolkit will be to improve the theming abilities (which in theory will improve look and feel cross platform), animations for swishy interfaces, and proper canvas widget/window. It will be remaining C which is excellent news for the millions of easy language bindings.
It should be interesting to see what happens, hopefully only concentrating on the GUI rather than a lot of the other toolkits which are a bit "kitchen sink" will do for it.