SpeedBox displays the current speed that you're traveling at by utilizing the iPhone 3G's built in GPS receiver.
You can switch the displayed units between km/h and mph.

Please note that due to the iPhone's limited GPS reception quality the application works best at speeds from 20-30 km/h upwards. The "Quality" indicator should at least show "medium" reception quality, while you will only get a accurate readout at "High" quality.