There are many I2C devices available for you to experiment with, here are some interesting examples:
- Dallas DS1338 - Real Time Clock w/ 56byte NVRam, used in the ZipIt RTC example.
- Dallas DS1621 - Digial Thermometer & Thermostat
- National Semiconductor LM75 - Digital Thermometer
- Philips PCF8574 - 8 Bit I/O Expander
- Philips PCD3312C - DTMF / modem / Musical Tone Generator
- Linear LTC1710 - I2C / SMBus Dual N-Channel Switch
There are far too many I2C compatible devices to list here.