The free GTK+ multi-platform toolkit can be used for creating graphical user interfaces.
Ranked in these QuestionsQuestion Ranking
Pro Nicer user interface when using PyGi
Creating cross platform enterprise apps with PyGi and with the help of Gtk Themes makes them look great.
Pro Language bindings
Pro Working with GTK focused Vala is great
Vala is made to make GTK easy [as well as for other things] and it really does, and makes it easier than working with C/C++, Python, or any other,
Con Terrible Integration
Looks and feels like an alien on all other platforms than GNOME.