The X-51 or WaveRider is an experimental flight testbed developed by a joint consortium of DARPA, NASA, Pratt & Whutney and Boeing. This scram jet scoops up air and compresses it into an ignition chamber. It is believed to have traveled at speeds over 4000 miles an hour for extended periods of time. It is launched by dripping it from an altitude of 50,000 from a B-52. A solid rocket booster pushes it to Mach 4 and then the scramjet is engaged where it accelerates to Mach 6.
That is probably the faster atmospheric vehicle on Earth.
Source: Wiki, YouTube