Derive 6.1 for Windows - free licensed download

[carey]

Yes, Derive is a mature CAS and of particular interest to users of TI CAS calculators.

Just to add an open-source alternative to the proprietary CAS systems mentioned (Maple, Mathematica) is Maxima, which like Derive (and Reduce) had its origin in the early CAS days (when it was the proprietary MACSYMA) and is actively developed and available on many platforms.