Komodo is a Mozilla-based cross-platform cross-language IDE created by ActiveState.
See http://www.activestate.com/Products/Enterprise/ASPN_Komodo/ for more information.
Information about other 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".