If you have an iPhone or an iPod touch, the XM Skydock easily transforms either device into an XM satellite radio for your car or truck. 

The XM Skydock gets its power from a car’s cigarette lighter adapter and charges while you listen.  You can listen via FM radio with the built-in FM transmitter, or if your auto has an AUX-in port you can connect via the included patch cable.  The XM Skydock app [iTunes link] work with the dock and lets you tag music for purchase in iTunes, set favorite channels and track artists and songs. 

Works with any iPhone or iPod touch running the iPhone OS 3.x.  Includes a magnetic mount antenna.

Amazon.com has the XM Skydock for $35 less than MSRP.