A venerable, if aging, GUI ToolKit originally developed for Tcl. The combination is known as TclTk and allows rapid, portable GUI development, as it is available on many different platforms. Bindings to Tk have also been written for many other languages, such as for Perl (Perl/Tk) or Python (Tkinter).
While more modern ToolKits such as GTK and Qt are nowadays the focus of attention, Tk remains the most easily deployable choice for cross-platform GUI programming.
wxWidgets is a younger attempt to achieve a similar degree of portability, by providing a layer over native ToolKits for which portable GUI applications can be written, but it has yet to achieve even a fraction of the ubiquity of Tk.