New applications can be created easily by writing short scripts for the windowing shell wishx, which provides access to TclX and TkX, an extended form of Tk. Note that the unextended windowing shell wish is not available with SGITCL.
Tk was written by John Ousterhout, and is described in his book, An Introduction to Tcl and Tk. The Tk widgets are probably the biggest reason for the popularity of Tcl, because they constitute a high-level GUI programming language.