When I first opened the book Tcl/Tk Tools, the extensions that caught my eye were Incr-Tcl and TSIPP. TSIPP is a graphical toolkit for use with Tcl/Tk written by Mark Diekhans. When I first used it I was enchanted by the realism of the images that could be created, but I found that writing code to create them was very laborious - it needed a GUI.