A megawidget is a widget that is implemented in Tcl, not in C. It should behave according to the principles set out by the standard Tk widgets, being able to be manipulated by invoking methods on its pathname, being constructed in the standard way, supporting options through configure and cget methods, participating in geometry management, focus traversal, event handling, etc.
snit provides a framework for creating megawidgets. I don't know whether there are any megawidgets actually in there, but certainly people are using the snit code to build them.
LV: there are certainly examples here on the wiki of snit widgets. Perhaps the authors would consider working with the tklib maintainers to have them incorporated into tklib? What about other widgets or procs that would provide a Tk developer useful tools? Any authors of such software willing to visit http://tcllib.sf.net/ and submit Feature Requests to have your software included? Anyone a fan of any specific Tk related procs/packages that would be useful for developing Tk applications? You could contact the author to see if they would be willing to allow their code to be included in tklib.
NEM 2004-02-26 1 : I am currently working on a CSS parser, which will then lead on to work to create CSS megawidgets based around the text widget and the canvas (which is more work, and so much longer term). The CSS parser is pure Tcl, should be finished some time in the next month (it probably has about 2 days more work left + testing, but finding the time...), and I will be submitting it for inclusion in tcllib when finished. The megawidgets will probably be written using snit, and might find their way in to tklib at some point. Combined with tdom or TclXML these might make a decent alternative to tkhtml with support for more recent (XML-based) web standards. Much work to do though.