GTK, The GIMP Toolkit, was initially made for [The Gimp] but is now a very popular toolkit with bindings for a lot of languages.
Look & Feel
If you'd like to change the appearance of your GTK applications, learn Howto make GTK apps look nice.
If you are not using one of the major Desktop Environments and thus are unable to configure GTK using the respective configuration tools, you may want to tweak your GTK settings manually. This can be done by editing ~/.gtkrc-2.0 (which can be automated by tools such as cog or gtk2_prefs).
You might want to take a look at our Collection of Useful GTK Settings.