FreeBSD

Initial development of FreeBSD started in 1993, taking its sources from 386BSD. However, due to concerns about the legality of all the sources used in 386BSD and a consequent lawsuit between Novell (then owner of the UNIX copyright) and Berkeley, FreeBSD ended up re-engineering much of the system with the FreeBSD 2.0 release in January of 1995 using the 4.4BSD-Lite release from the University of California, Berkeley. The FreeBSD Handbook includes more historical information about the genesis of FreeBSD.
Website: http://www.freebsd.org