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.
JH: Komodo for OS X is now in alpha (Aug 2005), available for user testing. The full Tcl capabilities are provided.
RLH - I am trying it out. I did post a bug to bugzilla because I have ActiveTcl installed but when I went into the language parser, Perl, Python, and PHP were there but Tcl was grayed out.