Sage is an awesome computer algebra system. It’s designed to be similar to Maple and Mathematica – but best of all it’s free and open source software!
Despite what it says in the README.txt, it may not be supported on Gentoo but it does compile on Gentoo with only mildly problems. All solvable. Mainly it’s just Fortran libs. Google will help you.
It has an awesome web based worksheet system, so you can use it anywhere in the world. At the moment, there is no Windows support. (Who needs it..?)