Taotk is a TclOO megawidget framework, developed by hypnotoad.
Fossil repo: http://fossil.etoyoc.com/fossil/taolib
Taotk is currently used by the T&E Solutions Integrated Recoverability Model for all of its Graphical User Interface. The project is pretty large, on the order of 750,000 lines of C and Tcl code. It would probably be larger without the Tao and Taotk framework.