Trilife is a project designed for large groups. You solder a small board and plug it into neighboring boards, making a giant array of lights that interact with each other. Here is a simulation of an 800 board array in action. 
Here's a step-by-step guide  to building a board. Here's the Github page  that connects to the project repository and video. We also programmed a "kicker" board (that's always on the brightest state) to start up interesting patterns when the kicker board is connected to the network. A 118 Hz, 0-5V square wave from a function generator also works. Kits were built by GadgetLabbers, then by folks at LVL1 (Thanks!) with more available through spring/summer 2012. Questions? Email firstname.lastname@example.org.