Maxima can also be used from Common Lisp programs as a library. See the section Lisp and Maxima in the Maxima manual for more information.
Maxima is a GPL'd Common Lisp mathematics application for computer based algebra (an implementation of MIT's Macsyma system). Until recently, it was maintained by William Schelter at http://www.ma.utexas.edu/maxima.html . He passed away in 2001 and since then a group of developers co-ordinated around http://maxima.sourceforge.net have been working on it.