I'm considering getting the Gary Fong Lightsphere II.
I don't shoot flash often, but I'm pretty sure I'll want to have a good diffuser next time I do, so maybe I'll order this so I don't have to wait for delivery when the time comes.
(Here's a review of the Lightsphere.)
Admittedly, $49 (for the most basic version) is a bit expensive for what it is and does, so there are very cheap home-made options.
You know what I'm just realizing... one of the reasons I don't often use Flash is just because I don't have a diffuser! Just for one thing, I often photograph objects and stuff for this blog, but I'm torn between barely-adequate indoor lighting and harsh direct-flash. If I had a good flash sitting on my Canon 5D, with a good diffuser, this would be perfect for it! I'll do it, by gum.