XPP (XPPAUT is another name; I (Bard Ermentrout) will use the two interchangeably) is a tool for solving
boundary value problems, and
It evolved from a chapter written by John Rinzel and me on the qualitative theory of nerve membranes and eventually became a commercial product for MSDOS computers called PHASEPLANE. It is now available as a program running under X11 and Windows.
The code brings together a number of useful algorithms and is extremely portable.