I just came across this nifty gadget at Amazon called the Actron CP9135 Auto Scanner that claims to "unlock the information stored in your car's computer" so you can find out what is wrong with your car yourself. You can also use it to turn off the car's check engine light (and help diagnose why it came on in the first place). Unfortunately, it only works on models 1996 and later, which means that my much older vehicle is out.
For $70, I think this device is a great deal because it could potentially save you a significant amount of time and money on trips to your mechanic. It also increases the level of control and knowledge you have over the maintenance and repair of your vehicle, meaning you're less likely to get ripped off my unscrupulous mechanics. I imagine it could also help you identify problems before they get so bad as to leave you stranded. Finally, according to Actron, no car can pass a smog test if the check engine light is on, so if all that's standing between you and a successful smog test is the check engine light, now you have a fast and easy way to take care of the problem. Out of 66 reviews on Amazon, almost all of them are four or five star. The reviews contain a wealth of tips for how to get the most out of the scanner.
Note: Manufacturer's Stock Photo