Another good algebra system written in lisp is Axiom (http://en.wikipedia.org/wiki/Axiom_%28computer_algebra_system%29 http://en.wikipedia.org/wiki/Axiom_%28com

Another good algebra system written in lisp is Axiom (http://en.wikipedia.org/wiki/Axiom_%28computer_algebra_system%29 http://en.wikipedia.org/wiki/Axiom_%28computer_algebra_syste...), which is interesting for another reason too - it's probably the best example of literate programming around.