If you have a tape deck in your car, and don't laugh, some still do, usually in combination with a CD changer. Scosche make a tape adapter that is $9.99 that they sell at Target and Wal-Mart, that will blow just about any FM transmitter out the water. Simply put the tape in, and plug it in via the headphone jack on your BB. Should sound really nice.
I just purchased a Jabra Cruiser from ebay - works very well - it's a bluetooth speakerphone (for handsfree calling) and it also allows you to stream music from your blackberry through your stereo via an FM channel.