ExpoImaging's ExpoAperture2 are $30 circular cardboard rulers that help you calculate proper depth of field when shooting with an SLR (digital or analog).
The ExpoAperture2 comes with two depth of field calculators--think of them like circular slide rules--one that covers wide and normal focal lengths (15-135mm) and one that covers telephoto focal lengths (70-600mm). The kit also comes with small plastic sleeves to store the discs, a reference chart to help you figure out the size of your camera's sensor or film, and a user's manual on CD. The manual includes a very good explanation of depth of field and how and why you'd want to control it. The result is a deeper understanding of photography that should benefit any but the most accomplished photographers.
How deep is your field? [Crave.CNET.com]