We've seen quite a few portable showers designed for keeping in cars. Most of the time, though, they're meant to go in the trunk or the boot, essentially requiring space you may not actually have to spare. The Road Shower solves this by mounting on your roof rack instead.
Since the water tank sits right on your roof, it's exposed to the sun the entire time, so they designed it such that the sunlight will end up heating the water inside (up to 115 degrees, depending on the weather). Yep, a warm shower waiting for you on the road without using up any of your propane supply. It can mount on any Thule, Yakima and compatible racks, with hardware available for other rack types (e.g. factory-mounted units).
The Road Shower is a pressurized water tank constructed with powder-coated aluminum. It can hold up to 5 gallons of water, which, if you ration them properly, should be enough to get you up to three showers while spending time in some backroads out of civilization. To use, you'll need to first pressurize the tank by hooking up a bike pump or a CO2 canister to the air intake valve (up to 15 PSI). From there, simply take the tethered hose from its mounting clamps, adjust the nozzle to one of seven settings, and send some of that refreshing shower down your body. It uses food grade parts for the hose and tank, so you can even use it to hold drinking water. Instant hot water for an afternoon tea for two, anyone?
Currently, the Road Shower is looking to make a bigger production run using Kickstarter to get backers. Available pledges to reserve a unit starts at $210.