What is a Ring Flash?
Ring flash is a circular photographic flash that fits around the lens, especially for use in macro (or close-up) photography. Its most important characteristic is providing even illumination with few shadows visible in the photograph, as the origin of the light is very close to (and surrounds) the optical axis of the lens. When the subject is very close to the camera, as is the case in macro photography, the distance of the flash from the optical axis becomes significant. For objects close to the camera, the size of the ring flash is significant and so the light encounters the subject from many angles in the same way that it does with a conventional flash with soft box. This has the effect of further softening any shadows.
Ring flashes are also very popular in portrait and fashion photography. In addition to softening shadows, the unique way that a ring flash renders light gives the model a shadowy halo which is a common feature of fashion photography...as Wikipedia says...
This is actually what you'll get:
Click here for Examples
As i previously said, i've never used one, but i'm planning to, so i found some useful link on how to do-it-yourself!
All the credits go to the authors.
1) The Strobist.com flickr group full of DIY ring flash and lighting DIY
5) Ring flash for built-in flash
8) Finally a really well done blogspot about strobist infos