Aka as GNU's Lesser or Library GPL See http://www.gnu.org/licenses/lgpl.html for more details.
See also GPL. and http://www.opensource.org/licenses/lgpl-license.php
jmn 2004-01-28 In light of the recent switch of the popular MySQL client library from LGPL to GPL http://www.infoworld.com/article/04/01/16/03OPstrategic_1.html ..
..it seems that some people at least view it as OK to use the LGPL as a stepping-stone to the more restrictive GPL license once their project has gained a certain momentum/mindshare.
While these sort of license shennanigans can in theory happen with any project, however it is currently licensed - I think this case serves as a warning against relying on the continuity of broad user support for projects which use such an overtly 'political' license as the LGPL.
The Tcl/Tk community has in general been very understanding of and even supportive of people choosing to release their own code (for extensions/packages etc) under whatever license they see fit, with merely a recommendation to developers to consider releasing their code in as open a way as possible.
The core Tcl/Tk code is presumably in no danger of being tainted intentionally with licenses that restrict the use of Tcl by it's significant commercial userbase, but lest anyone argue that the LGPL is 'harmless' and would be reasonable to include on a par with BSD/MIT style licenses; I hope that without starting a rant-war here, I'm at least able to give people a bit more pause to reconsider their use of it in the Tcl sphere.
As a side note:
Perhaps, a decade on from the 'Tcl war' http://www.vanderburg.org/Tcl/war/ , it's not considered sporting to harbour or encourage any sentiment even on this wiki that may appear to be anti-FSF/RMS (?), but I am surprised that such a fascinating piece of Tcl (and usenet) history as the 'Tcl war' appears to be quite absent in wiki discussion - I can't even find a reference to it. Is this intentional? Is it perceived as a 'negative' issue for Tcl and perhaps people worry it will even now scare off new users?
I've noticed that some people find raising 'licensing issues' in software forums to be 'annoying' - and as it has the ever present potential to explode into rants and flamewars this isn't really surprising; the 'noise' can quickly overwhelm the core topics of discussion. There do however remain people (newcomers to programming, people from countries with fewer lawyers?) who are not so aware of the controversies and how they relate to a particular programming community such as that of Tcl/Tk. I hope therefore that the odd discussion or note such as this can remain in relatively 'out of the way' parts of the wiki like this, without troubling too many people who may consider it all somewhat obnoxious.