The Scosche PassPort HomeDock ($39) adapts older, iPod accessories such as speaker systems and 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 HomeDock measures 1 inch by 1.25 inch by 2.75 inches, and is designed to fit snugly into any iPod dock that uses Apple's universal dock standard (for example, docks that accept those little plastic dock inserts that came with your iPod). Docks and speaker systems that use their own proprietary dock design (such as the Harman Kardon Go + Play) aren't going to work with PassPort. The same goes for speaker systems such as the Altec Lansing iM7, which limit the height of the iPod (the PassPort adds an extra half inch to the dock).