Dynolicious
http://gizmodo.com/5030749/iphone-ap...formance-meter
Looks quite interesting. Couple of motoring sites have done reviews and the consensus is that it's remarkably accurate.
Going to have to download this when I get home and try it. Not bad for $12.99, and uses the accelerometer to perform the calcs. Though, race chrono is free
http://www.dynolicious.com/