This information doesn't actually seem to be out their anywhere on the world wide internets. So hopefully when people google "bike bangkok bicycle free bikes smile bikes green bangkok"... they'll find this information. It's current as of January, 2011:
Biking in Bangkok is generally not common nor for the faint of heart. But there is a safe and tourist-friendly option: Bangkok Smile Bike. The program began in 2009 under the name Bangkok Green bikes, but was discontinued before restarting in 2010 under its current name. Free bikes are provided at 12 locations in the city for visitors to ride along two bike routes, one on each side of the river. At a very leisurely pace well suited for seeing the city, each route takes little more than one hour.Here are pics from the west-side ride (wwwest siiide!):
Bike lanes are provided for the length of the routes—sometimes on the street and sometimes on the sidewalk—but, Bangkok being Bangkok, these lanes are not respected as dedicated bike lanes. Frequent obstructions should be expected from cars, pedestrians, vendors, markets, and, after 3pm on school days, multitudes of uniformed school children. It is more polite to bike around Thai people walking in the bike path than to ring the bell and expect people to move out of your way. Inevitable, because of crowded vending areas and blocked bike lanes, some riding will be with traffic in city street. As a result, some comfort with city biking (and biking on the left side of the street) is advisable. But the streets on the route, at least by Bangkok standards, are generally manageble both in terms of traffic levels and speed.
To take a free bike, a passport or official looking photo ID is required (but not held). Bikes may be returned to any Smile Bike location along the route. Maps are also provided free of charge and mark the route and major sites along the way. As of January, 2010, hours of operation are Monday to Friday 10am to 6pm (though you may be asked to return the bikes by 5pm) and Saturday and Sunday from 10am to 8pm.
The bikes—small one-speed bicycles with hand brakes, bell, and a front basket—are generally in good condition. Beware, however, that the seat cannot be raised above a certain level and the bikes will not be comfortable for tall people. Locks may be available on request, but riders are asked not to stray from the well-marked bike route. Bike helmets are not provided, but with the slow pace, the danger is not much greater than walking along the same streets and sidewalks. Surprisingly, perhaps because of the rarity of bikes in Bangkok, bicyclists are given more courtesy from drivers than might be expected. And thumbs-up from tuk-tuk drivers are a frequent and pleasant surprise.
East of the river, the route forms a continuous circle and bikes can be picked up at Smile Bike locations on Snati Chai Prakarn Park, Saranrom Park, near the river at Chang Wang Luang Pier and Chang Wagna Pier, and Din So Road by Bangkok City Hall, south of the Democracy Monument.
West of the river, the route is mostly (but not perfectly) well marked and more of the ride will take place in the street. There is also a staircase or two to traverse, but the ride west of the river is extremely interesting as goes through non-tourist parts of Bangkok. Some of the side trips (marked on the map but not on the pavement) are particularly interesting. The route is one-way and starts at Somdej Phra Pinklao Road at Phra Pinklao Bridge across the river from the Bangkok Tourism Division office. They may run out of bikes late in the day. The second Smile Bike location west of the river is accessible by ferry, near the Thonbury Railway Station Pier and the old Thonbury Train Station (not to be confused with the actual Thonbury Train Station, AKA: Bangkoknoi, which is about 800 meters inland from the pier, and passed on the route). The bike route ends to the south at Wat Phichayayatikaram Worawihara. The penultimate west-side Smile Bike location is at the Memorial Bridge, which can pleasantly be walked over.