Thanks to Nunya Biznas for this link to a comparison of flash modifiers showing the effects of many different diffusers, ranging from bounce flash to umbrellas, Lumiquest softboxes and the Gary Fong Lightsphere.

The page was last updated in January 2006 so two noticeable exceptions are the Demb Diffuser and the new Gary Fong diffuser, the Whaletail, but it's a good resource all the same. (Sorry Gary but does anyone else think the Whaletail looks a bit like a toilet?)

Personally I've used the Stofen Omnibounce (good) and the Lightsphere (excellent) but nowadays I'm back to either direct or bounced flash. I've found that the most important qualities in a light source are direction and power, so if you've got enough light coming from the right angle then diffusion doesn't make a great deal of difference, at least not for the sort of photos that I do.