I have been using a cheap (<$20) OBD-II dongle in my car for over a year now. The Torque app[1] has an impressive feature list and constant updates. I use it with my tablet inverted on the dashboard during night-driving for giving me a heads up display[2]. It's also great for things like acceleration graphs, tracking stats over the internet, and keeping check on the engine's performance/mpg.
[1] https://play.google.com/store/apps/details?id=org.prowl.torq... [2] http://www.youtube.com/watch?v=4OPH7ZJJHV8