The free GTK+ multi-platform toolkit can be used for creating graphical user interfaces.
Ranked in these QuestionsQuestion Ranking
Pro Language bindings
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 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.
Con Sucks for traditional applications
GTK3 added CSD widgets and added a lot of paddings to make widgets bigger for touchscreen use, GTK4 also even removed Menubar and Toolbar widgets.
Con No menuicons or mnemonics
Gtk3 has removed the support of icons and mnemonics making navigating menus just terrible for the use with a keyboard. Sure there are some hacks to make them working again but they will never work as the did/do work in windows, gtk2 or qt.
Con Promotes wayland as the standard X11 server on unix
Lets face it Wayland is not a replacement for xorg since wayland works currently only in GNOME and there are many issues. Some desktops and window managers will never get ported to wayland and it is not available to all Unix and Unix-like systems. X11 on the other hand is available for almost everything so it will be available for the next decade or even longer.