Two which come to mind were the turret traverse system of the Infantry Tank MkII (Matilda. Prior to that no US tank had power traverse), and the Merlin Engined P51. Indeed the original P51 was built to British specifications, and the US wanted nothing to do with it.

They were sufficiently impressed by the Ordnance, Quick-firing, Anti-tank 6-pounder that they pressed it into service under local manufacture as the infantry's standard anti-tank gun, the 57mm Gun M1

Oh, and of course, they used a couple of Hobart's vehicles, the Sherman DD and, I believe, the Crab mine-flail, but I might be wrong on that latter.

After the war, they much appreciated the Vickers two-axis stabilization system of the Centurion tank. Actually, the entire Centurion as a whole impressed them.

The USAAF had several fighter groups equipped with British Spitfires during WW2. http://spitfiresite.com/2010/04/uncle-sams-spitfires.html

The comparable US aircraft - the P40 and P39 - were considered unsuitable for combat in NW Europe so the RAF talked the Americans into using Spitfires instead. USAAF units in the Mediterranean theatre also used Spitfires.

The US Navy used Spitfires for artillery spotting over Normandy.

Pending the arrival of the P61 Black Widow, Nightfighter Units used Bristol Beaufighters.

As a follow-up, what the the USN make of the Type 93 'Long Lance' torpedo?