Hoya R72 Infrared Filters
The Hoya R72 filter works by blocking all visible light - literally, if you put this filter to your eye, it will look black and totally opaque. However, near infrared light will pass through, and many digital cameras are sensitive to this light and will record a long exposure image showing blue skies as black, and trees and bushes with bright white leaves!
Nikon and Pentax SLR's work very well as do some of the Sony compact cameras like the V1 and V3