There are gadgets such as ScanGauge , DashDaq and many software based (including Iphone) diagnostic tools which connects to your OBD II ports via cable or wireless (wifi/bluetooth). These gadgets can display readings such as rpm , speed, coolant temp, fault codes, fuel consumption and many other data which are store in the ECU, including reset fault codes.
The following is an Iphone based software, check out the following link.
http://www.devtoaster.com/products/rev/
Garmin has come out with similar functionality which uses the bluetooth feature of the GPS unit to connect to the bluetooth enabled dongle connect to the OBD II port. And the GPS's display can display many engine data and more. It will cost USD150 for add-on which is called EcoRoute HD.