pbd

pbd is an ODE solution package, written in C, and based on the PBD method of van Bokhoven (IEEE Trans. on Circuits and Systems, v CAS-22, no 2, Feb. 1975, pp109-115). pbd provides a callable function, pbd(), which is intended to be called from user supplied code as part of a larger simulation or computation. It consists of fairly simple code, and is released under the GNU General Public License:
Find pbd at: http://www.onion-switch.com/physics/pbd/index.html