My car's "Check Engine" light came on.

This happened last year around this time, it was a false alarm on the "fuel filler cap left open" thing. I hadn't left the filler cap off, but that's the code that came up. I asked the mechanic to look at it while I had the car in for something else, so he just cleared it and sent me on my way.

Since right now, the only thing wrong with my car is the check engine light, I would not be getting any value by taking it to the mechanic. The guy on the phone said it was a minimum of $78.00 just to plug the thing into his computer.

If it really is another false alarm on the fuel filler cap sensor, I don't want to spend 80 bucks every time this happens. I know I could just use the Click and Clack Handy-Dandy Black Tape (TM) but I also want to be sure it's not something more serious.

So now I want to get a device that I can plug into the car myself and read/clear the codes. There has been plenty of discussion on this topic with regard to building the feature into the empeg, but I'm just looking for a quick, cheap, off-the-shelf solution, preferably something I can interface with my PC to make it cheap as possible.

Anyone have any experience with these kinds of things? Suggestions?