This is not an unix question, however, I'll just post my links to this:

[1] GTK+: http://www.gtk.org/
[2] Qt (commercial): http://www.trolltech.com/
[3] Lesstif (unix only): http://www.lesstif.org/
[4] wxWindows: http://wxwindows.sourceforge.net/
[5] V: http://www.objectcentral.com/
[6] FOX: http://www.cfdrc.com/FOX/fox.html
[7] fltk: http://www.fltk.org/
[8] JX: http://www.newplanetsoftware.com/jx/

Where [4] is my personal favorite, and the only one which I really
tested. It has a very good and complete documentation, and is
indeed an old project, which means its stable and complete. It is
however NOT AT ALL small.

[5] might be what you're looking for, relatively small, and
portable