for a "true" cross-platform toolkit though I've never used it.
GTK is slighly less user friendly (in my opinion) but is "lighter" than Qt. It is also a good choice for programming in pure C.
Qt is highly object-oriented and a natural choice for C++ programmers.
Qt is platform independent and so is GTK. All three are available as bindings for a number of scripting languages like Python, Ruby, Perl etc.
For Python there is a very light toolkit based on Tcl/Tk called Tkinter
which works on both Windows and Linux and is installed by default on Windows along with Python.