These look nice, but I'm going to give the same comment that I give to those looking for ways to use their graphing calculators: make sure you know how to do these kinds of problems before you use these utilities.
-Dan
Here is a site with several online numerical math tools:
Numerical Mathematics Tools
The tools are for immediate online use, although I suppose they could be saved for offline use as well (since they are written in Javascript).
Some of the tools include utilities for solving the Quadratic, Cubic, and Quartic Equations; solving N Equations in N Unknowns; Eigenvalues and Eigenvectors; and more.
Regards.
David