I've been toying with i2c for avr's (both slave and master) for a while. I somehow managed to get a SPI slave working (data sent via parallel port, through the programmer, gah! Kudos to avr for intelligent programming interface design though).
I had some notes on i2c for atmega16, which I might make a tutorial at some point (not now for sure, since i'm having exams).
Are you guys looking for a generic i2c/spi tute, or a specific implementation?