I have a Snap-On MT2500 that I use, but I've seen some simple OBD II code readers from Harbor Freight for under 50 bucks.
There's a couple nice programs that can turn your laptop into a very powerful scan-tool. Ease Diagnostics
www.obd2.com and AutoTap
www.autotap.com are two of the most popular. I do like the fact that you can make live movies of your datastream with these, much more so than you can with a scanner like my Snap-On. Since you're working with the processor speed and memory of a real PC, they really have some excellent graphing and data recording capability.