Sure, probably lots of them.
To name a couple:
Many PDP-11 Unixes and BSD Unixes, see
http://minnie.cs.adfa.edu.au/PUPS/index.html
(a source license costs $100, CD-ROM is ~$5)
FreeBSD Unix (descendant of 4.4BSD Unix), see
http://www.freebsd.org
CP/M (almost complete), see
MAIN SITE AT            : 
http://www.devili.iki.fi/cpm
PRIMARY US MIRROR AT    : 
http://www.mathcs.emory.edu/~cfs/cpm
SECONDARY US MIRROR AT  : 
http://CPM.INTERFUN.NET
Heath HDOS for the H-89 used to be available, is it still?
I'm sure other list members can chime in with other systems.
Dave
Jgzabol(a)aol.com wrote:
 Hi all,
 question: Is there any classic operating system known for which the
 complete source code is available ?
 Thanks and regards
 John G. Zabolitzky