byteparadigm.com has a nice introductory article on I2C and SPI protocols:
Today, at the low end of the communication protocols, we find I²C (for ‘Inter-Integrated Circuit’, protocol) and SPI (for ‘Serial Peripheral Interface’). Both protocols are well-suited for communications between integrated circuits, for slow communication with on-board peripherals. At the roots of these two popular protocols we find two major companies – Philips for I²C and Motorola for SPI – and two different histories about why, when and how the protocols were created.
Introduction to I²C and SPI protocols – [Link]