If you want to use the TFL bike hire scheme, but want to ensure that the nearest docking station has a bike, and that there will be space at your target destination, there are a number of tracking apps available.
This is the nicest I've seen, since it's overlaid with a map, has a nice graphical interface and gives you a real-time update of the number of bikes at each docking station in a very clear and easy to interpret fashion.

http://www.oobrien.com/vis/bikes/