There was a download of SPiM 1.13 on the developer's website when we last checked. We cannot confirm if there is a free download of this software available. SPiM lies within Education Tools, more precisely Science Tools. The most popular version of the tool is 1.1. The actual developer of the free software is Microsoft Research. The most frequent installer filename for the software is: spimplayer.exe. We recommend checking the downloaded files with any free antivirus.
From the developer:
The Stochastic Pi Machine (SPiM) is a programming language for designing and simulating computer models of biological processes. The language is based on a mathematical formalism known as the pi-calculus, and the simulation algorithm is based on standard kinetic theory of physical chemistry.
You may want to check out more software, such as SPIM for FS9 or PCSpim, which might be related to SPiM.