Many extensions are available.
Tk is an extension developed by the creator of Tcl and used for creating scripts that interact with users through windows. Tcl can act as a Unix shell and Tk was originally developed to create windows for the X Windows environment. The Tcl FAQ . is maintained by Larry Virden. Mike Hopkirk maintains a site at SCO which also has a comprehensive set of links. Mostly these are highlights of the comp.lang.tcl newsgroup.