I just picked up a Scangauge II, it reads all the codes, lets you view all the sensor outputs, and give you a trip computer. I've been driving with it for a couple days now and I love it.
Right now I have it mounted on the steering column, but I'll probably end up integrating it into the center console, or mounting it above the rear view mirror.
If you go over to clubtitan.org, they have a groupbuy going on right now for 149 shipped. Mine showed up in about 3 days.