An 80/20 bouncer requires a ceiling low enough to bounce off. Luminquest makes a kit that includes an 80/20 bouncer, along with the stuff to make it into a softbox with some different reflectors and a diffuser, etc... It all fits in a wallet, and it pretty handy. I've carried one for years.
I also use the Sto-Fen Omni Bounce and the Lightsphere II. They are both variations of a translucent plastic diffuser that fits on the end of the flash. They bounce diffuse light all over the place.
The Sto-Fen is small and convenient, and does a great job. Easy to carry all the time. When you turn the camera vertical, and swivel the flash to bounce off the ceiling the Sto-Fen presents a much smaller lightsource to the subject than when the camera is horizontal.
The Lightsphere II is huge and obnoxious, and many photographers claim it's too silly for practical use. It totally looks like tupperware, but I like mine. It has a cap that will pop off the end to allow bouncing from high ceilings. It's too big for me to carry all the time, but I use it at weddings and portrait shoots. The Lightsphere presents the same size and shaped light source to the subject in either position.
Some photogs will point out that it's probably pretty easy to make your own mini-softboxes, diffusers, bouncers, etc... I'll show up at a wedding and shoot a K-1000, but I don't have the guts to show up with a margarine tub duct taped to my flash. On the other hand, I don't have the guts to use the Lightsphere when I'm in street clothes.
