When you're taking photos with a flash, there's a problem: Because the flash is mounted on top of your camera, you get some pretty horrible shadows. One way around that is to move the flash as far away from the lens as you can, in the now-famous Strobist approach to photography lighting, but there's another way, too: Move the flash closer to your lens. A lot closer.
A ring flash is designed to be ring-shaped, and to sit around the lens of your camera. Originally developed as a tool to facilitate dental photography, it's great for all sorts of macro work. However, it quickly got another use, too: in portraiture and fashion.
Because the light from a ring flash is coming from exactly the same direction as your lens, you don't get the harsh shadows traditionally associated with flash photography. Perfect!
The only problem is that ring flashes can be very expensive. A Canon MR 14EX ring-flash will set you back the best part of an arm and a leg (that's about £400), and even third-party ring flashes will cost you at least a couple of fingers, if not a hand.
The El Cheapo Deluxe option is to use the strobes you already have, and use a light shaper that'll turn your normal directional light into a lovely circular light source. There are loads of ways to make your own ring flash; Out of a plastic bowl (better instructions here), with some bits of plastic, with a whole load of light sources or even using a metric tonne of white LEDs.
Whist it's lovely to spend a rainy morning up to your elbows in glue and bits of aluminium foil, there's an easier option as well; DIY Lighting Kits does a flat-pack kit that's quick to assemble, and pretty easy to use, too.