Under what license(s) are Tcl and such accessories as Tk, Expect, tcllib, ... available? Is it possible/practical/easy/... to use Tcl in commercial products? Is the Tcl license compatible with GPL?
The answers are ... [very liberal, and so forth]
Each author is free to use whatever license s/he wishes. The TCT encourages people to use a BSD related license similar to the one used in Tcl/Tk/tcllib/tklib/etc. as some companies find it less restrictive.
The license for the standard Tcl distribution is
http://purl.org/tcl/home/software/tcltk/license_terms.html .
[lots more details]