Bluetooth Handsfree Car USB MP3 Player Built-in FM Transmitter Rearview Mirror with Wireless Headset Black
Since it is illegal to drive and use a cell phone, this solved a huge problem for me.  My boss likes to talk while he drives and now, when he calls, I don't have to worry.  My only complaint is that I had to decide which phone to use with it.  Most of the time I use my work phone, but on weekends and on trips I will be using my personal phone.  Would be sweet if you could program multiple phones to it!