Sage is a free mathematics software system. It is built out of nearly 100 open-source packages and features a unified Python interface. There is a nice video introduction to Sage by William Stein.
Sage 5.0 is available

Sage is a free mathematics software system. It is built out of nearly 100 open-source packages and features a unified Python interface. There is a nice video introduction to Sage by William Stein.