All cars sold in the US after 1996 have a standard diagnostic port, typically under the dashboard by the driver's knee (a chart tells you where to find it on your car). It's the thing that the mechanic connects to when performing a state inspection or performing certain types of diagnoses. It's this port that the CarMD device connects do. Just plug it in and turn it on to begin.
After a few seconds, one of three indicator lights will glow. Green means everything is A-OK, yellow suggests a pending failure and red means there's a problem that needs immediate attention. Once that's done, you pull the device from your car and plug it into your Mac via USB. From there, it launches a browser which connects to their database, tells you what's wrong and suggests a course of action. Pretty neat.
If it's something as simple as a loose gas cap (this will trigger your check engine light), you can save yourself the cost and time of driving to a garage. Otherwise, you can print the diagnosis and tell the mechanic exactly what the issue is, saving labor costs. Additionally, the CarMD database has access to up-to-the-minute service bulletins, something typically reserved for dealerships.
I'm no gear head but I see the appeal of this device. Anything that can save me some dough is welcome. CarMD is on sale now for $98.99US.