Car Audio FM Stereo Transmitter/Thumbdrive MP3 player ($36.95!) Looks pretty slick: Plugs straight into lighter socket for power, accepts music via USB port (via thumbdrive or other storage) or via 1/8″ audio plug (for CD/DVD/MP3 player), and transmits to FM.
About the only weakness I see here is that (like many of the cheaper FM transmitters) it is limited to certain pre-set channels at top and bottom of dial. But for $40, not bad!
Link from Scripting News