Two auto performance meters for iPhone
I've seen a lot of fun accelerometer uses in the App Store so far, but this is probably the coolest -- Dynolicious will actually use your iPhone's accelerometer to determine all kinds of stuff about how awesome your '92 Subaru is, from 0-60 and 1/4 mile time up to lateral Gs and horsepower. Just throw your iPhone somewhere secure in the car, hit go, and Dynolicious will track all that hardware stuff for you (even over multiple runs). Very awesome -- it's in the App Store right now for $12.99, which is pricey, until you see what an actual GTech meter will set you back.
What? Gas is super expensive right now, and you want it even cheaper? Wish granted: our friends at Autoblog have also dug up an app called g-tac, which will track your times and even graph them out for you. Unfortunately, g-tac won't do the cool accelerometer stuff, measuring out your lateral gs and horsepower for you, but you know the old saying: you can have things fast, good, or cheap, and you only get to pick two.
