XPPAUT is a tool for solving

* differential equations,
* difference equations,
* delay equations,
* functional equations,
* boundary value problems, and
* stochastic equations.

The code brings together a number of useful algorithms and is extremely portable. All the graphics and interface are written completely in Xlib which explains the somewhat idiosyncratic and primitive widgets interface.

http://www.math.pitt.edu/~bard/xpp/xpp.html