Games
Magic Pen - loads of physics
Fantastic Contraption - and here too
Science Friction - guide an air puck around a table
Shifty the Robot - Very simple, good for little kids
Operation Asteroid - Asteroid shoot 'em up
Serious Physics - Quite tricky - using a combination of circular motion and Gravity
Electronica Island - Produced by plugin2engineering. This game takes you through the tasks needed to be a engineer in the energy industry
Bio Dome, Bridge Building, Colour Matching - Three games from the Smallpeice Trust
Cargo Bridge - Another great engineering challenge (thanks to K.F. for sending the link)
Sumdog - Some fun maths games
Lasers & Mirrors - From Cartoon Network
Lunar Lander - from Phet, great fun
Electric Field Hockey - from Phet, difficult
Gravitee 2 - Golf in space
Launchball - a nice one from the National Science Museum (Physics/Engineering challenges)
Gravitas -