Next time you’re in Paris, Barcelona, Stockholm, Frankfurt or Oslo, forgo the metro and get some fresh air instead. With “bike sharing” you can rent a bicycle to cruise across town.

Bike sharing is an easy-to-use new service that is exploding in Europe: 10,000 bikes in Paris, 1,500 in Barcelona, 2,000 and an additional 3,000 to come in Copenhagen. With a swipe of a card, you unlock a bike from any station and ride it across town. Once at your destination, you steer to the closest bike rack and, with one more card swipe, return the bike to the public network. You pay less than $.50 for the trip, and the bike is once again available for the taking.

In Paris, Velib’ will rent you a bicycle. Since July, more than 10,600 bikes have been posted at 750 stations, and the numbers of both will double by year end in Mayor Delanoë's effort to make Paris more green. The great news is that City Hall has made sure the service is convenient for travelers, not just Parisians, by offering short-term passes and access in eight languages. A one-day card is available for €1 ($1.36), a weekly card for €5 ($6.80) or an annual card for €29 ($39.70).

In Barcelona, Bicing has 1,500 bicycles available at 100 stations and plans to have 3,000 by year end. Like many bike-share programs, Bicing offers its users a Google mash-up, with the number of available bikes at every station updated in real-time.

And Bycyklen will rent you wheels in Copenhagen. Riders deposit a 20 kroner coin into the bike handlebar to unlock it from the bike rack. Bikes may be rented for unlimited amounts of time, although they may not leave a designated area.

Enjoy the ride!