For I2C level shifting this circuit from Philips is awesome: App note AN10441.pdf and an97055.pdf. I built one last weekend and it's working well. Two 5 cent mosfets, 4 resistors, done. I used 2N7002P mosfets, 3.3K pull-ups on the 3.3V side and 4.7K pull-ups on the 5V side.