$0.99 from Steven Troughton-Smith
Speed is a drop-dead simple speedometer for iPhone that uses the location data from the GPS in iPhone 3G to provide a near-accurate representation of how fast you’re actually traveling.
Featuring lush graphics and a clear, legible display, Speed is the perfect bicycle, boat or train speedometer. Both kilometers and miles per hour are supported, and a landscape mode for widescreen operation. Even more, if you double-tap the dial it will invert itself – perfect for reflecting in windshields!
As this works using the GPS, you will have to be moving at a certain minimum speed for it to register, and from testing it seems to be 5km/h. Ergo walking won’t normally make the dial update unless you have super-accurate GPS coverage.
iPhone 3G is required for this application.
The much improved Speed v2.0 is coming soon for $1.99, but if you buy the current version now for 99¢ you will be upgraded for free to the new version!