Scriptics of course , The reference. Mktclapp,
by Richard Hipp,
a very simple tool to create mixed Tcl/Tk/C/C++ applications.
There is a lot of well known extensions for Tcl/Tk, let name
[Incr Tcl],
Tix or
BLT,
but i prefer to speak here about light tcl extensions , able to be integrated in standard tcl environment.
Richard Hipp, again, create some interesting extensions
We can find some others at Brian Oakley tcl page.