I've been using an app called Torque for a while now and wasn't sure if it was common knowledge around these parts. I originally set it up to diagnose some engine issues (turned out to be fuel contamination in the end) but along the way found this app and now use it everyday for displaying extra info about the car while I'm driving, using my phone as a display. Noticed that some people have used a USB OBD2 adapter with a russian program called FF2, this uses the ELM327 interface too.
Lots of info and screenshots below about all the different features:
You can buy the bluetooth adapters all over the place, I got mine from ebay. I can't remember the particular seller where I got mine from, just make sure it is v1.5 onwards e.g. [url="http://www.ebay.co.uk/itm/ELM327-OBD2-OBDII-CAN-BUS-V1-5-Bluetooth-Auto-Car-Diagnostic-Interface-Scanner-/120978389221?pt=UK_Diagnostic_Tools_Equipment&hash=item1c2adfb8e5"]http://www.ebay.co.u...=item1c2adfb8e5[/url]
You can get mini versions but mine is an original full size one and fits in the OBD2 port in my focus mk2 24/7 and I never knock it.
If you already have an android phone and have it mounted in the car when you are driving, its a no-brainer.
Will put pics, more details, app config stuff etc if anybody is interested