I've used several wxWidgets apps and they're all really, really nice. Qt seems too big and GTK+ doesn't play well with Windows from what I can see (not that it's that good on Linux either - I don't like how the whole project is handled).
However, one thing I've noticed is that wxWidgets apps seem to be a bit slow to start (minimum 1 second or so), so I'd test that carefully for lightweight apps.
Name:
Anonymous2009-11-23 21:23
I really like Qt overall, although it has its fair share of bad designs and unnecessarily limited APIs. I'm also not perfectly satisfied with its documentation completeness; it seems to cover everything you need to know, but it doesn't always answer specific questions (does event X occur before or after some visible state is changed? Can I use Y safely in situation Z?), but to be honest no other extended toolkit I know does it significantly better, except maybe the java API.
I don't have experience with GTK+ or wxWidgets. I do have experience with Swing (java) however, and I'd personally and very subjectively rate Qt higher.