Whether you're tackling mountain trails or just trying to get through Manhattan's Chinatown, rough terrain can put your bike's tubes at serious risk. It's not usually an option to hop off and adjust your tire pressure, but a company called Adaptrac has a system that mounts a toggle right on your handlebars and allows you to raise or lower your pressure even while riding.
The Adaptrac system is essentially a CO2 cartridge of varying sizes that straps to your downtube, where you'd mount a water bottle. It comes with replacement hubs of its own, and plugs into both front and rear tires. Controls--the toggle for adding/reducing pressure, as well as a pair of analog pressure gauges--are mounted on the handlebars. Assuming it all works--and there's a lot of engineering trickiness going on here, to make sure everything stays connected and rotating properly--you'd be able to individually adjust tire pressure while riding.
That could definitely come in handy--Adaptrac notes that you could decrease the pressure for more traction on a downhill coast, and then increase it upon the next climb for an easier ride. It'd probably match nicely with a powered gear-set.
No word on pricing or availability yet, as it's still in the prototype stage, but that should come fairly soon.