From Hackaday: A Simple Method for Expanding Arduino IO Capacity

This is an example project entry that directs the reader off-site.

Personally I'm a fan of the arduino platform. I do realize that it's a crutch and real-men us PIC or whatever. I've done simple things with PICAXE, but the turn-around time and approachability of the arduino is unmatched. Some may consider it a toy, but it's a fun toy, and with it I can quickly turn out a physical computing project.

Here is a simple technique using I2C to link two arduinos.

More at hackaday here or at the original source