My limited knowledge on the subject is:
- As for cross-platform, there seemed to be some effort for a while going into a Mac OS X Cocoa port of GTK+, but I don't know the current status. I think the main company behind the port may have gone under. Relevant links may include the following:
- Tcl/Tk itself seems to be still going pretty strong. They've got nice theming now. Python is still using it as their default bundled GUI toolkit. The Perl/Tk dist may not have been updated for a while though.
- Wx seems huge, multi-layered, and the API resembles MS MFC, IIRC. My guess is that people use it primarily because (A) they're familiar with MFC, (B) it's an easy-ish cross-platform solution that provides native widgets (which, I presume, they think their users would like), and (C) it's actively maintained. Python users seem enamored with Wx. Personally, I was disappointed to hear that Padre went with Wx, but I'm just an armchair quarterback on that.
My advice (taken with at least a cubic cm of salt) would be to go with GTK+. It's cross-platform ports are in alright shape, it's not as bulky as Wx, it doesn't have the Tcl/Tk dependency, it's fairly well-maintained, and it's probably the closest of all the GUI toolkits to being considered the "standard" one (though none really are, of course). Also, everything Zentara said.