I just bought a Nspire CAS Touchscreen for $50, the best calculator available. With the 3.2 firmware update, Nspire CAS has more features and math algorithms than TI-89 Titanium. I previous bought a Nspire CAS in 2009.
is free CAS for Windows which has all the features of SAGE in
universities, yet is a lot smaller filesize. It is based on
DOE-MACSYMA. Its abilities include symbolic integration, 3D plotting,
and an ODE solver.
I found a free CAS running on Ubuntu (1.5 GB) in a Virtual Box image.
This "Sage" software might exceed Nspire CAS functionality and its
free. I know Sage is used by universities who can't afford commercial