If you're looking to add a Bluetooth connection to electronic gear you already own, check out the Kinivo BTC450 car adapter kit and the Grace Digital 3play Bluetooth Receiver. Both get the job done for a reasonable price.
The Kinivo sticks to your dashboard and adds Bluetooth capability to your car's stereo system via the Aux port. It's unobtrusive and stays in place until you're ready to remove it.
The device is powered by the cigarette lighter port. There's a pass-through USB port so you can still charge your phone. The BTC450 has limited capabilities, but it works perfectly at what it aims to do. Once you've paired a phone, it automatically reconnects whenever you turn on the car. That works so well, in fact, that you're going to have to remember to turn off Bluetooth if you're planning to listen to the radio instead of streaming from the phone.
You can answer calls by pressing the button on the Kinivo and there's a built-in microphone. You can pick one up for around $35 and, while it's not substitute for a built-in Bluetooth system, it's a great option for anyone who's not getting a new car soon.
The 3Play is a Bluetooth receiver with a twist: you can connect up to three devices at the same time and, when one device is playing music, either of the other two devices can take over the stream just by hitting play. Having DJ battles at a party could be a great time but this is definitely the kind of device you'll want to keep away from a group of young kids who like to argue. The device remembers up to 7 paired devices, so it's good if you've got a house full of phones and tablets.
Once you've charged, you've got 8 hours of battery life and it's so small (about 2 inches square and about 1 inch thick) that it's portable enough to use in your car (even though it doesn't have the speakerphone capabilities of the Kinivo). The 3Play uses Bluetooth 4.0 connectivity with aptX, so the audio quality is good. You can find one for about $42 online.