FLTK, if you are serious about lightweight.
http://www.fltk.org/
edit:
Blurb from the website:
FLTK is designed to be small and modular enough to be statically linked, but works fine as a shared library. FLTK also includes an excellent UI builder called FLUID that can be used to create applications in minutes.
I'll add that its mature and stable, too.
