<img src="http://tctechcrunch2011.files.wordpress.com/2007/01/MMC flash cards and USB-enabled storage devices like a thumb drive or a USB hard drive. When you plug in any of these, the player will stream music over FM to your car or home stereo, albeit only on one of 7 frequencies.
The reader and transmitter is powered by your car's cig adapter or 7 AAA batteries. When you're at home, you can use a regular AC adapter. You can grab one now for $59.99.