A large amount of free and open-source software is distributed under the GNU General Public License (GPL). All of my software is.
The GPL is not an “end-user” license in the same way that licenses for commercial software are. This is because the GPL license actually affects modifying and distributing of the software, not use. GPL-licensed software can be used for any purpose without restriction. Its purpose is that any new software which is derived from (or simply just a modified version of) existing GPL software can only be distributed under the GPL.
However, authors of some GPL applications are so used to having to “accept this license” fed to them at installation time by commercial applications, that they have introduced a check box in their installers requiring the user to “accept the GPL” in order to use their GPL software. This is Bad and Wrong.
Two examples I noticed recently were Website Baker and Mambo, both web page content management systems. I was testing them out and hit “Check this box to accept the GPL”, without which the software could not be installed. So, in the great tradition of open source software, I decided to submit this problem as a bug report to both sets of developers. This gained one favourable response and one ongoing and mixed response. In fact, the developer for Website Baker had read my bug report, responded to it, fixed the code, uploaded it into the version-tracking system and closed the bug within a few hours: excellent, I’m glad I reported it.
Actually, in both of the above cases, it would be perfectly legal for anyone (me, you, whoever) to download the software and modify it (without installing it), and change the installer so that it does not require the user to accept the GPL during installation!