Distributed under the BSD license, which has very few restrictions on usage. See ftp.freebsd.org, ftp.netbsd.org and ftp.openbsd.org and the various mirrors of these sites. As part of the GNU project, the FSF has implmenented versions of most of the usual Unix commands, plus lots of great utilities. Available from and numerous mirror sites around the world. Linux A Unix-like system distributed under the FSF's General Public License, which guarantees availability of source code.
Many different distributions exist.
They operate a branding system based on a specification and a set of conformance tests. The Common Desktop Environment. Various Linux-related newsgroups. SGI systems comp.sys.sun. Newsgroups relating to SCO Unix.