Hi Martin,
* Martin Sluka martinsluka@mac.com [2013-01-16 14:42:16 +0100]:
http://skim-app.sourceforge.net
MacOSX
Unfortunately Skim is not entirely Free Software. Only the front-end of Skim is available under a Free license, the back-end (its rendering engine), which does most of the work of reading PDF files, is under a separate, proprietary license.
Statements on sourceforge about Skim's licensing only refer to the parts which are Free Software; sourceforge does not host all parts of Skim that are required to make it work, including those parts which are proprietary.
This is why we do not list Skim on pdfreaders.org, although we have been aware of it for some time.
Regards, Matthias