<img src="http://www.lumiquest.com/images/softscreen.jpg">

~$12 iirc

I can't tell if that's for the pop-up flash or not (probably not), but you can buy a smaller, more compact one for about $15 that fits ontop of the external flash.

I know the Omnibounce has a following, but if you're just using the built-in flash, I guess this is as good a solution as you'll find. Gary Fong's lightspere is all the rage now with external flashes.