An implementation of the 3D reversible cellular automata developed by Ed Fredkin and Daniel B. Miller in their 2005 paper, Two State, Reversible, Universal Cellular Automata In Three Dimensions. A preprint is available here.
The new paper, which makes numerous references to the website running this software, is up on the arXiv.
It's running live (as of March, 2012) here.
More info can be found in the faq.
All of the code is right now in index.html.