Éric Lévénez created and maintains an excellent, detailed chart showing the history of UNIX through the marriages, divorces, births, and funerals of its family members. It includes events up through late September 2001. It’s a great reference both for history and for seeing the major influences on current UNIX-derived OSes. Print it out on 13 letter-size pages and tape it up on your wall. His site has similar charts showing the history of Windows and the history of computer programming languages, also very fun and informative.