Julian's Great Idea #163:
For me to play my MP3s or stream music (Pandora, Slacker Radio, etc.) in my car, I use a small Chinese-built FM radio transmitter that plugs in my phone's audio jack and sends my phat beats to an unused frequency on my dashboard radio.
It works fine, but its a lot of cables and stuff. Plus the transmitter is hella weak, it's gotta sit right on top of the stereo to hear anything but static (it was only like $3).
What I need is a smarphone, or just an app, that takes out the middle man, and lets me transmit via FM frequency directly to the car's stereo.
PROS: Good for us people with high-tech phones but low-tech cars. Good for hands-free calling too.
CONS: Might be a battery sapper.