Well, if you're just interested in using it on your own cars to
familiarize yourself with how it works, you'll want to get one that is
compatible with your make and year vehicle(s). If you drive older
vehicles, a scanner with older software can be just fine.  They're all
over eBay. I bought an OTC Monitor 4000 last year. If I were doing it
again, I might choose a Snap-On MT2500 instead. The other thing to think
about is upgradeability. If you think you might want eventually to use the
scanner on other makes and models, make sure you buy one that can easily
and inexpensively be upgraded with different software (usually a different
cartridge).

I'm happy with my AutoTap.  The new ones can do all OBD-II models.
(I've only tested mine with GM products.)

Unlike the snap-on that Dan recommended, it doesn't do ABS systems, just
the OBD stuff.  However, for the basic package it's only $200 US.  I
doubt the Snap-On is... but for a DIYer like me, it's all I need and a
whole lot more.  (reset MIL, realtime scan, data log.)

You do need a laptop or a Palm Pilot.
(www.autotap.com)

Do a google search on OBD-2 or OBD-II. There are several on the market. Keep
in mind that most of these scanners scan the mandated EPA information but do
not read the manufacturer specific information. Some read more than others.
I use the cable at www.obd-2.com with my laptop. Their software is regularly
updated. If you want to get into the manufacturer specific information you
will have to step up to the more expensive units.....