Komodo is a Mozilla-based cross-platform cross-language IDE created by ActiveState.
See http://www.activestate.com/Products/Komodo/ for more information.
download for registered users only/FREE evaluation copy.
Note carefully the platforms supported - ActiveTcl/Tcl Dev Kit runs on more platforms than does Komodo.
Information about other development environments can be found at IDE.
Among the many interesting particular features of Komodo is the "ActiveState GUI Builder", a "simple tool for creating Tk-based GUIs for Perl, Python, and Tcl".
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.]
IDE ]