Simbrain

SIMBRAIN is a freeware tool for building, runnning, and analyzing neural-networks (computer simulations of brain circuitry). Simbrain aims to be as visual and easy-to-use as possible. Also unique are its integrated "world component" and its representation of the network’s activation space. Simbrain is written in Java and runs on Windows, Mac OS X, and Linux.
Find Simbrain at: http://simbrain.sourceforge.net/