http://sf.net/projects/incrtcl/ is where the CVS for iwidgets is maintained.
In the early life of itcl/itk/iwidgets, they were all three distributed in the same source tar file.
During 2001, a TIP was submitted proposing that the incr tcl be included as an extension distributed with Tcl.
If/when this proposal is implemented, users will need to download the tcl, tk, itk, and iwidgets tar files to get the pieces to use iwidgets.
See incr Widgets for a list of the widgets included in the set. [What do we do about the cases below which overlap with widgets that already existed in Tk, etc.?] In the case of overlap, just update the associated page with info about each of the items, and the extension with which it is associated.