STk takes Tcl and Tk (8.0 in STk versions 3.99–4.0.1) and replaces Tcl's internals like Tcl_Eval with calls to a Scheme interpreter. STk has been superseded by a new Scheme interpreter called STklos , which implements R5RS and comes with GTK+ bindings for creating GUIs. Somewhat confusingly, "STklos" was previously used as the name of the CLOS-style object system in STk.
|... STk knew its "hour of glory" in late September 1994, when Richard Stallman of the Free Software Foundation posted an article to comp.lang.tcl newsgroup titled Why you should not use Tcl which conducted ro a nearly one month long flamewar ensued. Richard Stallman proposed in this article to use STk instead of Tcl. After that, the FSF started the Guile project, which still use the object layer of STk as the basis for its own object layer.
|— Erick Gallesio (links and formatting changed from the original)