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 than GPL or LGPL.
The license for the standard Tcl distribution is
http://purl.org/tcl/home/software/tcltk/license_terms.html . Kbk writes in comp.lang.tcl the following summary of the Tcl license:
In even sinpler language:
Paragraph 1. It's ours.
Paragraph 2. You can use it, give it away, or sell it. But
only if you tell people about these terms.
Paragraph 3. Don't sue us if it doesn't work.
Paragraph 4. We don't have to fix it, either.
Paragraph 5. Even if the Government uses it, we can use it
ourselves, give it away, or sell it.
[lots more details]
It is recommended that before you begin working with an extension or application you take a look over the license included. If a license is not available, you may want to attempt to get some sort of license in writing so as to ensure that you don't encounter any surprises in the future.
If you are a software author, you should think carefully about what kind of end user license agreement (or EULA in legal parlance) you use. However, as David Welton mentioned recently in comp.lang.tcl, http://www.dina.kvl.dk/~abraham/rants/license.html makes a strong argument against developers trying to invent their own licenses.