Velib, the Paris public bike rental service is a success, but a unique problem has emerged: users are dropping more bicycles in lower neighborhoods than riding them up. In order to give incentives for the cyclists to drop bikes in "high" stations, the company will give you 15 additional minutes to drop the 22 kg bike off in "high" neighborhoods such as Belleville or Montmartre. Velib has already more than 183,000 users, 1,451 stations available and 20,000 bicycles.
If you're a really fit biker, you can try to participate in the Velibmontant contest, in which you have to climb up rue Ménilmontant riding a Velib. The current record is 3 minutes 15 seconds. The hill is a steep one!