A Neat Place for MonkeysHowStuffWorks.com is one of the most interesting sites I have found. Briefly, it describes how various items, like electromagnets and digital cameras, work. I particularly enjoyed the explanation about water guns.
In case anyone is interested, you can register for the site and newletter. If you do, be sure to use firstname.lastname@example.org as the referring email so I can win big prizes. :)