Alpine has just released its DPR-RDS1 which allows users to add iPod and iPhone playback to any car stereo with an FM stereo (basically all of them).
Features include a 2-megapixel camera, built-in FM radio with RDS, Bluetooth stereo A2DP, and A-GPS.
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The Kensington RDS FM transmitter works well enough, but unless you find RDS data necessary (and you have an RDS-compatible car stereo), you should consider another model.