wxMaxima is a GUI for Maxima, a computer algebra system. Basically, wxMaxima + Maxima lets you type in various algebra statements, and it can respond with SYMBOLIC answers. So it can accept (x+4)^3=7, and then "solve for x". GPLv2.
LOTS of people use math, so this is a handy tool.
More details at: http://wxmaxima.sourceforge.net
