The source for iwidgets can be found at the http://sf.net/projects/incrtcl/ project CVS.
Also, see http://incrtcl.sourceforge.net/iwidgets/ .
See some online documentation (which may be dated) at http://incrtcl.sourceforge.net/iwidgets/manpages.html . http://incrtcl.sourceforge.net/iwidgets/iwidgets/iw_hierarchy.html is documentation for the megawidget hierarchy (again, this doc may be dated).
The incr widgets distribution includes:
LES on 11-aug-2008: I just tried Iwidgets for the first time and for the first time also had version problems using Tcl/Tk:
package require Iwidgets version conflict for package "Tcl": have 8.5a6, need 8.4-8.5 :-(
RZ There is a itcl4 only itk replacement at http://chiselapp.com/user/rene/repository/itk A slightly modified iwidgets is at http://chiselapp.com/user/rene/repository/iwidgets
To run you need at least itcl-4-0-b8-rc. Binaries are at https://sourceforge.net/projects/kbskit/files/itk/