I have an ALDL cable, available here:
http://aldlcable.com/
Hooks up to my laptop's USB port, and to the OBDI connector. I use TunerPro software, which is free (and open source, I think). Got this setup to hook up to my '94 Chevy Corsica. It's an oddball system, a so-called OBD1.5- and most cheap scan tools won't work with it. Several other free software packages come with that ALDL cable, but TunerPro is the only one I've found that will work with my oddball system. It will monitor sensor data and switch inputs, and check and clear codes- both for the ECU and the ABS system.
Software available here:
http://tunerpro.net/
In more popular applications, this can be used for re-flashing, emulating, and the like. Not much tuning support available for a '94 Corsica, though.
The definition files are created by random people (many of the files are posted on the TunerPro site.), and are therefore a little buggy on occasion (especially the ABS definition). But it's to be expected in a low-volume free software. It doesn't cover all vehicles, so I'd suggest doing some investigation before you go for this setup.
I haven't tried hooking up to the wife's OBDII vehicle- it will require a different cable (available from the same aldlcable.com site). But for my application, this is hard to beat. I've no doubt that TunerPro and/or some of the other free softwares will work with the wife's car... I'll investigate sometime.