FriCAS - an advanced CAS. I've lost track of it a bit - FriCAS is formerly Axiom, a quite extensive mathematics package like Mathematica, written in Common Lisp. Available for various systems, somewhat rough in the interface (just a command line), but very powerful. And somewhat more modern than perhaps Maxima (whereas for Maxima there are GUI options, I have no idea what FriCAS has to offer). Like every decent open source project, it is of course only a fork of a fork - under Open Axiom there is another project and under Axiom the original code is continued. Although for me, since Sage (and actually even much more since Anaconda and IPython Notebooks), all of this has become less relevant - in general, my requirements are at a much lower level and are already covered by the libraries available via IPython and Python. Of course, Mathematica and similar are still interesting - but the motivation to start the packages and to enter the rather different syntax world is rather low. Python has long since achieved the good-enough status.