I'd suggest looking at an Actron code reader. They have a wide range at reasonable prices. I have the CP9135. If I recall correctly, I got it for less than $100 on Amazon. If your cars are post 1995, an OBD 2 (also referred to as OBD II) scanner is all you need. Make sure the check to see what vehicle models are supported (e.g., domestic, foreign, specific brands), as there are occasionally some variations from scanner to scanner. The most important thing IMO is the ability to read and decipher codes for you. Features like live data, downloads to PCs etc. are nice to have's, but may not be worth the extra money. A scanner's a great investment for troubleshooting error codes and for re-setting MILs on modern cars. All DIY mechanics should have one.

One option if you don't want to buy one: At least in the States, most auto parts stores (e.g., Schucks, Pep Boys, AutoZone, etc.) will let you borrow their scanners for free and use them in their parking lots. This might be a suitable option if you have a one time or time sensitive need to check/clear codes.

 

dcmc, I have a reader that I use my laptop to pull the data from the car. I can set up a complete screen to see all the data in the car by selecting what I want to view. The problem that I see with having access to all the data the computer looks at is this: do you know what the normal value for that parameter is and if it isn't at that value, is it acceptable? So, if you have that knowledge, then spending some more money on a data logger/code reader will be worth it. The system I used I picked up a few years ago for $150 (USD) and was capable of reading all 3 types of system out there (Ford, Chevy, All others). Check out http://www.obdii.com, they have a bunch of stuff that you can get through the site and even have systems now that you can use a PDA to access your car's data.

 

I use one on my laptop as well, it came from Alex Peper and can be purchased at http://www.obd-2.com/ It works on all 96 and newer cars. I get regular updates as well. I think you can view some demos online as well. I have been very pleased with this.