I was surfing around -- you know, the usual sequence of Slashdot, Groklaw, random linked articles -- and encountered the LLVM license (actual license text). I thought I would take a moment to look at this one and compare it to other permissive Free Software licenses. Broadly speaking, the LLVM license is one that allows everything, and requires: retaining copyright notice and disclaimers, in source or in documentation, and disallow using authors names for endorsement. Compare it to the 3-clause BSD license , 2-clause FreeBSD license or the 1-clause MIT license.
A couple of comments on this license family is in order; one is that I find the MIT license a tad unclear(!) because I don't understand how to include a copyright and permission notice that is part of a comment in a source file in the software. The intention is clear enough, I guess: put the notice in a README or at the end of the software manual, and you're clear. It seems to me that some mention of binary distribution vs. source should have been done, if only to clarify that point.
The second is that the header of the LLVM license invites a form of poor copyright management; this isn't the license's fault per se, but it contains language that suggests to other developers to do things sub-optimally [[ gosh, it's hard to pick just the right words here; "wrong" sounds more pithy, but is also more likely to annoy people into not listening at all; the point is there are best-practices ways of doing things and anything else isn't, well .. , the best ]]. It's the inclusion of a group of developers at the top -- the "Developed by" line, as well as the "Copyright
(c) 1996-2008 The KDE System Monitor Developers
(c) 1999-2008, The KDE Developers
(c) 2003, The KHelpCenter developers
(c) 1998-2000,2003 The KFM/Konqueror Developers
(C) 1999-2008, The Konqueror developers
Program copyright 1997-2001 The KInfoCenter Developers
The problem lies in the fact that these groups are defined if and only if you have access to information outside the sources themselves -- e.g. mailing list archives or version control system history. Putting these non-existent groups in a copyright header weakens the copyright (just a little -- after all, each original author is a rightsholder, regardless of whether he or she puts her name to it) and makes compliance engineering just a little more difficult. Note that putting an existing organization there that actually holds the rights is just fine: my own code in KDE SVN should read "Copyright 1999-2008 KDE e.V." because I used a Fiduciary License Agreement to assign the rights. Again, none of this is the license's fault per se, it's just an easy-to-misconstrue example.
So here it would be better -- for everyone, and KDE coders in particular -- to follow an example that said "Copyright
Finally, the last bit of commentary goes not to the license text but to the explanation given by LLVM for their reasons for choosing this license over the GPL -- except for llvm-gcc, which is necessarily GPL-licensed because it is a derived work of gcc, which is GPL licensed itself. And it's the use of the word "viral" that bugs me here. It's bolded on the LLVM license webpage, and is wholly unnecessary since they manage to explain what the GPL does pretty darn well; it's just adding a typical FUD-word to an otherwise fine page explaining a license choice (a legitimate license choice for a Free Software license done by the original authors, and hence one to be respected). A better line for that particular web page would be "any code linked into llvm-gcc (which is GPL licensed) must also be released under the GPL, as per clauses 2 and 3 of that license." (This assumes it's GPLv2-licensed).
Anyway, an interesting (for me, but then I like to read licenses and the reasoning behind license choices) jaunt into non-copyleft licensing territory. [[ PS. And yes, there is a 4-clause BSD license, which has the Advertising Clause; I'm not aware of a 5-clause one, but there is a 3'-clause license, the Sleepycat license, which is formatted like a BSD-style license but has a strong copyleft component. ]]