Here is a page that links to several good mathematical tools:
http://www.akiti.ca/Mathfxns.html
These utilities offer quite powerful, though specialized, mathematical capabilities and I think they fit into the philosophy of the Portable Freeware site quite well: they are free, they are for immediate online use, and since they run within a web browser, they work on all the operating systems. (In fact, since the utilities are written in Javascript, they can be saved and used offline too.)
Among the tools offered are utilities for solving the Quadratic, Cubic, and Quartic Equations; solving N Equations in N Unknowns; Eigenvalues and Eigenvectors; and more.
Regards.
David