Originally uploaded by motoed.
In an article I wrote for istockphoto, I discussed a little ringflash reflector that I have been experimenting with. The above photo is a shot of that setup... and here is the quoted text about this lighting setup.
For a "poor man's" ringflash, you have a couple options. It works best with two flashes, but you can make do with a single flash unit. Cut an oblong hole (vertically) in a large white card just large enough to fit over the outside of your lens. Set your flashes up on a stand so the flash units are on either site of your lens, facing each other (I use Canon flashes that operate as master/slaves via IR). Then place the card over the lens through the hole in the card and position it so the top of the card is tilted at a 45 degree angle toward the subject. The flashes will bounce off the card and toward your model and give you an effect "similar" to expensive ringflashes. You'll have to experiment with power output and exposure, but you can get some interesting lighting effects.
See the article for more information and some samples.