The Scosche PassPort ($29) adapts older, iPod accessories such as car stereos and speaker docks, to work with the iPhone and recent iPod models. Specifically, the PassPort steps down the voltage of older accessories from 12 volts to 5 volts, to address the fact that the iPhone and newer iPods models have ceased support for 12-volt charging.
The PassPort is essentially a plastic cable adapter (female to male) that attached to the end of the dock connector you wish to correct. The adapter measures a little more than an inch high, making it a little top-heavy for use in an iPod charging dock or speaker dock (Scosche sells a more expensive HomeDock version for this purpose).
Ostensibly, the PassPort is aimed at people who have an iPod-compatible car stereo that uses Apple's older 12-volt charging standard instead of the latest 5-volt standard. These car stereos can be expensive to replace (if not impossible), and are often factory-installed, making the prospect of a $29 cable adapter much more appealing than a stereo upgrade.