We are down to my Miata, and when I get a job that may be a problem here. Plus her trunk isn't very large and that creates problems for us as well.
ETA: http://www.flexcar.com/ is another option, but also not in my 'hood yet.
Second edit: http://www.eartheasy.com/live_car_sharing.htm
An overview of car-sharing with links to car-sharing companies and a list of areas they serve.