Science Struck
Science Struck: Simple Machine: Levers
Explains what a lever is, its history, how it works, the formula for calculating its mechanical advantage, classes of levers, and uses of levers.
Integrated Publishing
Integrated Publishing: Levers
A quite lengthy chapter on the simple machine the lever. Includes uses, classes, mechanical advantage, formulae, etc. A full discussion, with good illustrations.
The Franklin Institute
Spotlighting Simple Machines
Simple machines such as inclined planes in wedges and screws are explained.