One of the simplest gadgets ever is also one of my favorites. I'm talking about Griffin's original AirCurve, the acoustically-shaped amplifier that docks and increases the iPod/iPhone's volume. It's now been updated for iPhone 4s, increasing music 10dB louder.
Working specifically with iPhone 4s, it no longer sits the iPhone in the docked position, instead just resting it neatly inside (with a rubber area underneath to protect it). There's also now a metal kickstand for holding it either vertically or horizontally, and luckily the price hasn't changed either—it's $20. [Griffin via GearLog via OhGizmo]