I2C Bus for ATtiny and ATmega

Step 7 Web Resources

Here are the links to the datasheets for the parts used for the experiments. You should definitely get these if you get nothing else.
Port Expander
Ram
EEPROM

Being the creator of I2C, NXP (Philips) has a ton of great stuff. (They like to use square brackets in their URLs, so I can't properly include them here. Sorry.] To get to the I2C area, select Interface from the Products list. You'll be able to get to their I2C site and access to all the datasheets and apps notes they offer. The I2C bus description and technical details in particular are here.

Get the ATtiny2313 and ATmega168 datasheets (data books?) from Atmel.

The Atmel application notes are here. Look at AVR310 and AVR315. Grab the code also.

Have a look here for lots more I2C stuff.