Science! -> metric
Weight (froot 'n' veg) -> imperial (lbs & ozs)
Height (people) -> feet and inches
Height (buildings, giraffes) -> feet or yards
Speed (cars, Usain Bolt) -> mph
Area (e.g. internal, flooring etc.) -> sq. feet
Area (e.g. fields, counties) -> acres, sq. miles
Weight (whales, elephants, your mum) -> tons (imperial)
Weight (people) -> stones 'n' pounds
Weight (hashish, grass) -> ozs and fractions thereof
Volumes (any) -> metric (cubic cm, metres, kilometres)
Mars Climate Orbiter -> mixture of metric and imperial -> roflcrash!