GTK is the leader. Not easy or fun, but can probably do anything you want it to.
Qt is easier.
Tk is too simple. It's like BASIC - good to get started but you'll soon outgrow it. Though there are Megawidgets to fill in missing functionality.
Still worth playing with:
Fox toolkit
fltk
wxWidgets
On Apple, Cocoa descends from a LONG line of R&D going all the way back to NeXt and HyperCard and The Mother Of All Demos. (It was Apple that brought windows and mice to the people in the first Macintosh.)
>>15
PyGtk makes it very usable indeed. Far better than any Qt bullshit.
You don't need to write your UI code in a compiled language anyway. 98% of the time you're waiting for user input in the event loop anyway, and the other 2% of performance critical code you can write as a C library, then hook into it with ctypes. It's by far the sanest and easiest way to design a program.