Sage install for Ubuntu is very straightforward--they even provide the command to unpack. If you've programmed in Python, or even better Fortran or C, the installation is mostly trivial.
Nontrivial is compiling from source, however :-).
Sage does more than Mathematica, as I understand it. It can be used as a replacement for Matlab, R, etc. I got informed of it by friend who is an applied mathematician by day and a .Net guru by night. Wikipedia has a decent writeup at: http://en.wikipedia.org/wiki/Sagemath
Nontrivial is compiling from source, however :-).
Sage does more than Mathematica, as I understand it. It can be used as a replacement for Matlab, R, etc. I got informed of it by friend who is an applied mathematician by day and a .Net guru by night. Wikipedia has a decent writeup at: http://en.wikipedia.org/wiki/Sagemath