This
other thread reminded me I've been meaning to mention this.
When I was considering whether or not to totally ditch my car*, the availability of zipcars in my neighborhood was just the push I needed to finally take the plunge.
Zipcars are basically hourly car rentals. You don't pay for gas, insurance, or maintenance. You reserve online or with your smartphone, and then you use an RFID credit card type thing held up to the windshield to unlock the car, which sits unattended. The keys are inside. You park it back where you found it when you're done.
When I ran the numbers on car costs for my old 97 wagon vs renting zipcars, the zipcars came out cheaper as long as I didn't drive more than about four hours per week, and that was only operating costs, not acquisition costs. For anyone who still commutes by car regularly (why are you doing that!) this isn't a good option. For people like me who basically only used their own car infrequently, paying to own a vehicle instead of rent one didn't pencil out.
As an added bonus, zipcars allow you to take a station wagon one day and a Prius the next, depending on your needs. Depending on what you normally drive and how far you have to go on a particular trip, it is sometimes even cost effective to rent a Prius for $7/hour rather than pay to run your normal gas guzzler.
If you're thinking of giving up your car but are reluctant to lose the flexibility for infrequent trips, consider signing up. I've found it totally worthwhile.
* note: since I got married, our family of four does have our own vehicle again. I still use zipcar whenever we really need to have two cars in use at the same time, which isn't very often.