From the MAKE Flickr pool:
Member Chris Metcalf constructed this flash snoot from corrugated cardboard and gaffer tape.
Although it may sound like the name of a Dr. Suess character, a snoot is a sort of choke used in photography to limit the spread of light from a flash. Snoot’s can create an eerie vignetting effect or dramatic highlight – and can be constructed from recycled household items.
DIY Snoot on Flickr –Link
How to make your own snoot:
Make a DIY flash snoot on DSLRBlog –Link
Black straws snoot grid on LightingMods –Link
DIY ring flash –Link