Maxima

Maxima is a computer algebra system.

Presentation
Based on Macsyma, a computer algebra system developed in the late 1960s at the MIT, it became free in 1998. Different graphical interfaces are available, such as wxmaxima.

Importing/Exporting MathML
Maxima contains two packages to deal with MathML that you can find in the directory /maxima/share/contrib/. The first one, lurkmathml, allows to export Presentation MathML whereas the second one, maximaMathML, can export both Content and Presentation MathML and import Content MathML. These two package are experimental, so a lot of work remains to do before they are really usable. See these Examples