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 ]
[WK] Komodo's main advantage in debugging is that it keeps breakpoints correct when editing a file. In contrast with TclPro (and probably Tcl Dev Kit), this means that you don't have to move the breakpoints after you edit the files.
[We could use more reviews and comparisons of Komodo and TDK.]