See http://www.activestate.com/Products/Komodo/ for more information.
download for registered users only/FREE evaluation copy.
Information about other development environments can be found at IDE.
See also GUI Building Tools.
Features specifically related to Tcl are
You can see recorded webcast demos of Komodo at http://www.activestate.com/Corporate/Events/Webcasts/ .
Apparently, it could also be AOL's new Web browser: [L1 ]
RLH - I just wish it ran on OSX.