I2C is a multi-master serial bus that's very common on microcontrollers and can be used by a robot builder to attach sensors or actuators, or as a communications channel between multiple processors. The uC Hobby blog has posted a good introduction to I2C by Grinan Barrett that includes examples, schematics, and source code for working with the I2C bus. In addition to taking the reader through his own experiences, there are also links to FAQs and other sites with additional information.