Dr. Hanley's Science Graphs  
   Enter first and second order non-linear ordinary differential in analytical form. The program will generate a numerical solution to the equations and graph multiple curves on a single graph. Can also be used for algebra and calculus formulas. Includes
   http://www.hanleyinnovations.com/sgstandard.html

FlexPDE  
   A general, script driven solution system for Partial differential equations, including equation interpretation, mesh generation, numerical solution and graphical output.
   http://www.pdesolutions.com

XMDS: eXtensible Multi-Dimensional Simulator  
   A code generator that integrates equations translating from XML to C++. Unix/Windows. Free to download under GPL.
   http://www.xmds.org/

Phaser Scientific Software  
   A Universal Simulator For Dynamical Systems
   http://www.phaser.com

Diffpack Online  
   An object oriented development framework for the solution of partial differential equations. Free demo CD. Online ordering.
   http://www.diffpack.com/