The City of Paris launched a cheap and very easy network of bikes to rent in 2007. You will find stations like every 500 meters (to pick them up, or bring them back), and if you use it for less then 30 mins, it's even for free.
Nevertheless you have to subscribe to a pass, which can be for 1 day only (1,70 €), 7 days (8 €) or a whole year pass, and you will have to secure the bringing back of the bike with a virtual payment of a 150€ deposit. (This will not be charged, when your bike is re-checked in within 24hrs.) You can simply use your bank-card to subscribe at any station.
Be careful that there is a WORKING bike where you check in, look at tires, chain, seat etc. - you will easily spot those with a default. And sometimes - in busy districts - it can be a bit of a hassle to find an empty parking slot, but every station has an interactive map to show you the nearest empty slot in the area.
It can also be hard to find some on top of Menilmontant or near Montmartre, cause the stations are up on a hill, and people are more likely to roll down then back up ;)
