This video shows the Sikorsky X2 breaking the world's helicopter speed record: 260 knots—that's 299.2mph vs the 172mph of conventional helicopters. Bonus: The video shows their X2 Light Tactical Helicopter too.
The X2 uses a single T800 engine to power both the rotors and the tail-mounted variable-pitch propeller, which provides forward thrust to enable the high speed. Other technologies include fly-by-wire flight controls and active vibration control.
Also, it's really pretty. [The Dew Line]