I've created a fairly extensive Instructable on the I2C Bus. It offers a complete set of drivers for the ATtiny2313 and the ATmega168 - probably pretty easy to adapt to others. There're schematics showing an I/O Port Expander, an SRAM, and an EEPROM with complete example programs.
The Instructable is here: http://www.instructables.com/id/I2C_Bus_for_ATtiny_and_ATmega/
Hope you find it useful!