Carsharing is quite a good thing. I'm actually using one in my own city (although, sadly, not regularly). One barrier to use is the monthly or yearly fee. A company in Paris has decided that this was unfair and has launched a set of carsharing tariffs that excludes montly fees or membership dues. The plan is called Mobizen and it's free to join; costs are €9 for to drive one of their available (located around the city) Mercedes A-Class for less than 200km.
Other options for carsharing in Paris are Caisse Commune, which has prices ranging €12 to €25 for the monthly fee, then €2-3 per hour and €0.21-0.44 per km. Then there's also Okigo, charging €9.90 a month, €4 per hour and €0.35 per km. Finally you can also rent a car for less than a day at Hertz, which offers 4, 8 or 12-hour rentals for €35. There's a demo video after the jump.