What do you get when you combine an unlimited data plan, a smart phone, an 1/8 inch cable, and the Pandora app? Yes, you guessed it...poor-man's Sirius Satellite Radio...well...kind of. Sure, it doesn't have all of the talk shows, real-time sports, and radio personalities, but it does do one thing pretty well...stream commercial-free music to your car at no additional cost.
If you have a BlackBerry or an iPhone, or other supported smart phone, you probably already have an unlimited data plan and so you are most of the way there. All you need to complete your project is a new application for your phone and an 1/8 audio cable. Let's start with the application.
Basically, there are several different choices of phone applications here, and we'll cover two. Pandora (pandora.com) is a no-cost commercial-free music streaming website that also has Blackberry and iPhone apps. Simply download the app, install it, sign-in, and bam...all the goodness of your Pandora stations streaming to your phone.

To get it for your iPhone...http://www.pandora.com/on-the-iphone
To get it for your Blackberry...http://www.pandora.com/blackberry

Another app is slacker (slacker.com).

To get slacker for your smartphone...http://www.slacker.com/products/everywhere
Now that you have music streaming to your phone, you just need to pipe it into your car's audio system. If you are lucky, your car already has an aux in that support an 1/8 inch stereo audio cable, otherwise you'll need to check into your car stereo's ability to have an external input. Next, just run over to Radio Shack and pick up a short cable, and run it between your phone and your car radio and done. Project complete.