The Kensington Pico Transmitter will work with any dockable iPod, but if you have a nano, this should really be your first and only choice. It's the same height, width and breadth as a nano, so perfectly preserves your player's smooth lines.
As with the iTrip, you pick your frequency using a rocker on one end, which you then press to confirm. The frequency is displayed using large digits on the iPod screen, so this process is much easier when driving with the Pico FM than with the iTrip, its closest equivalent. There are also two presets that enable you to switch to commonly used slots without taking your eyes off the road.
At £20 this is an extraordinary bargain and, as it's not tethered to a lighter socket for power, can also be used at home. As such, it's our top choice for FM transmission, although for longer, battery-sapping journeys, consider the Macally FMCup.