Gadget Alert! From the August 2006 Popular Mechanics:

Every car made since 1996 has an OBD-II (On-Board Diagnostics) connector underdash. It's where the mechanic plugs in his scan tool to check your car's health. A pro-grade scan tool can cost thousands. CarMD costs $90, which is close to what a mechanic would charge you to use his scanner to diagnose a problem. The CarMD uses the memory and screen on a Windows-based computer to display the data from your car's engine management system and to look up information from CarMD's Web site. There you can find an explanation of what those trouble codes really mean, and some insight into what might be causing them. carmd.com