Have you heard of the BubbleBum? I hope you have, since I posted about it before :) It's an inflatable booster. It is small when deflated so it can fit in a suite case, backpack, or even a purse.
There is also the Ride Safer Travel Vest for children over age 3. It positions the car's seatbelt properly on the child, but is safer than a booster for children that are too small or too you g for a booster. You must have a lap/should belt or a lap belt and tether.
There is also a new booster coming to the US called Boostapak. It's a backpack that is also a booster seat!