I recently started a project which required several microcontrollers (placed around the house) to be able to communicate with each other. The difficult part is choosing a communications method and protocol. After doing some research, I found the following to be suitable options for inter-microcontroller communications.
The most interesting and suitable option I found was was the RS485 bus. This bus allows for communications over potentially long distances (greater than 1.3 km), tolerates high speed, allows for many protocol designs, is asynchronous and easy to use. Several protocols allow for multiple bus masters with collision detection. Overall, the bus should be pretty reliable. Further research suggested that either CAT3 (phone cable) can be used with 6 contact RJ11 type connectors. This allows for full duplex communication over 4 lines and one ground line. CAT 5 ethernet cable with RJ45 type connectors could also be used if desired. However, twisted pair cable is strongly recommended (such as CAT3 or CAT5).
Another option is using ethernet. The Microchip ENC26J60 transceiver can be used. However, I’m not sure how ethernet can be used as a bus (without a hub/switch). More on this later.
If wires prove to be too cumbersome to implement, one may chose to take the wireless route. Futurlec sells SPI radio transceiver modules at a low cost. Implement your own protocol.
Other busses such as CAN, LIN and USB are further discussed on the following site: http://www.ucpros.com/