The S-PRES rare event algorithm is available for download [ spres_2.1.tgz ] as a C++ wrapper program for your chosen method of dynamic simulation.
Instructions for accelerating your simulation program using the S-PRES wrapper class are here. Full text of the paper which gives an in-depth look is behind a paywall at J Chem Phys, however you will find an earlier version of the paper on arXiv.
If you use the code (or try and fail to use the code) then please send an email to the address on my uni.lu page saying roughly what you used it for and how you got on. As well as being interesting with any luck, this will help me to improve my work, and to justify requests for funding in the future.
Thanks to Sven Dorosz, as of 2.1 a parallel build option for S-PRES is available, based on openMP.