i was after one aswell, i have never used one but i knew they were suppoosed to be good. i have just got;
http://cgi.ebay.co.uk/OBD-II-Kiwi-Wi...ools_Equipment
and its great, it doubles up as a code reader and also gives you live data about the car, iat, coolant temp, maf rate, mpg, and quite a lot more. The only catch is that you need to have an iphone/ipod touch.
this app, is a free one and is better than the one i have paid for. if you click the link it does explain a little more.
http://www.fuzzyluke.com/
and this app is the paid for app, it again, explain things better than i can!
http://www.devtoaster.com/
if anyone wants to know more about it, just ask me and ill do my best! im in manchester if anyone wants to actually have a look at it!
hope this helps someone!
cheers,