Zasim is both a framework/library and a usable program for experimenting with and developing cellular automata. It compiles cellular automaton loop code to C or runs it in pure python code. The cagen module offers a "building block" system for plugging different aspects of a cellular automaton together to greatly reduce the amount of boilerplate code that needs to be written.
It is written in python and offers integration with the various IPython features, such as the IPython notebook web application and the IPython qtcosole web application. It is also mostly compatile with pypy.
Code and website
The code can be found on github.