Tix (Tk Interface Extension), an add-on for the Tk widget set, is an
extensive set of over 40 widgets. In general, Tix widgets are more
complex and more capable than the widgets provided in Tk. Tix widgets
include a ComboBox, a Motif-style FileSelectBox, an MS Windows-style
FileSelectBox, a PanedWindow, a NoteBook, a hierarchical list, a
directory tree and a file manager.
Install the tix package if you want to try out more complicated widgets
for Tk. You'll also need to have the tcl and tk packages installed.