It isn't as elaborate as some of the readers people have here but I got a obdpro serial reader from a fellow jeeper, bought a brand new palm V with all it's accessories from eBay for a grand total of 15 bucks, got this free obd reading palm software and put it on, got a male to make serial adaptor and a null modem serial cable thing and now I can read error codes and a brief description if it has info, reset the engine light, see what the oxygen sensors are showing, directly see the mph, rpms, timing, engine temp and a whole bunch of other stuff directly from the computer.
And if you are someone that cares about speed it can time how fast it can go from 0-60.
Google USB/bluetooth/serial (whatever works for you) obd2 scanners
Incompetence in motion
96 XJ 4dr, 4.0, AW4, NP231,
87 Wrangler, 4.2, TF999, NP231