Ring flashes are circular flash units that mount around the lens and produce flat, even lighting. They’re generally used for macro photography and were popular for fashion photography in the 90s. As they’re quite expensive, these two articles on how to make your own are quite handy. Both have solid step-by-step instructions.
First is a how-to from DigiHack about making an adaptor rig for an existing flash unit. Cheap and easy.
Then there’s one from Robots Kick Ass on how to make a ring flash from scratch using LEDs.
Via MAKE Blog.