NeoK12 offers a set of YouTube videos explaining and illustrating the types of forces, including normal force, friction, centrifigal force, centripetal force, gravity, magnetism, buoyancy, and nuclear.  All videos and lessons on this site have been reviewed by K-12 teachers and are safe for kids.

Tinker Ball is a great interactive game for helping kids learn how simple machines can be made from everyday objects.  They use hockey sticks, funnels and baseball bats in creative ways to channel a ball from the top ledge to a cup at the bottom.  How many different ways can you help the ball reach its goal?  Can you create more than one path for the ball to take to the cup?