Gray Line Tours is a good deal. We paid $16 each for roundtrip airport shuttle service to and from our hotel.
The downside is that it's really difficult to get through to their customer service. You have to be patient and sit on hold. If you'd like some advice, don't think you can wander through hotels while you do this, my phone kept randomly cutting off.
They pick you up three hours before your scheduled flight to make sure you make it on time.
Bell Trans also offers a comparable service.