Old beater or the family car, your teen is going to want to make the space their own as much as they can. Not to mention make it more useful with technology.
Newer cars have auxiliary jacks and sockets built in, but it's possible to end up without any. Good options might be to set the car up with an iPod connector or change out the stereo to one with a jack so your teen can plug their own tunes in when they drive. (Road trip playlist, anyone?) And maybe an added benefit because it's more difficult to answer your phone while it's attached to the radio!
If you're sharing a car, a CD case is another nice option. They can take their music with them when they go, but not leave it cluttering up the car when they return. Maybe add a new CD or two inside.
I'm better these days about remembering to charge my phone, but I still carry a car charger with me just in case. Having a phone for emergencies doesn't help much when the battery's dead. Drives my husband batty. And if your teen is anything like mine, they're still learning the art of responsibility.
One of my favorite things - and likely what my niece will be getting - are personalized car mats. You can get all kinds of accessories for inside the car - from Hello Kitty to butterflies to zebra - but these are a unique touch. The ones I found at Personal Creations (shown in the picture) are just $29.99 for a set and come with an initial and name.
Oh - and to be ultimately practical, consider the kinds of things no car should be without - jumper cables and an emergency kit. Boring but absolutely critical.