Capital Bikeshare launched last month here in DC. It provides over 1,000 bikes at stations all around the city (concentrated in the denser parts of town), as well as in neighboring Alexandria, VA. Rides are free if they're 30 minutes or less, and charged by the half hour after that. You can join by the day, but it's much cheaper to get an annual membership, as I did, which also gets you a key fob that unlocks the bikes.
You might be asking yourself why I would join CaBi (as people have dubbed it) if I already have a bike. I originally signed up with the idea that it would be useful only a few times a month. Since getting my key fob in the mail a couple weeks ago, however, I've found myself using it more often than I envisioned. Because you can drop off the bike at a different station than the one you pick it up at, a key advantage is that it allows one-way bike trips. I used it when picking up and dropping off a rental car, for example. But it's also useful in instances where a bike trip is interrupted by other forms of transportation -- for instance, when meeting friends for happy hour, walking or taking transit to dinner, and then heading home. This can be awkward when I have my own bike along, but well-suited to bike sharing. Another situation where it's useful is when you're out and about and realize you want to go somewhere else that would be best reached by bike, as I did today. (This last scenario means you're riding without a helmet, which isn't ideal, but the bikes are big and heavy, with conservative gearing, which leads to cautious riding.)
An app on my phone shows the location of stations and how many bikes (and empty docks) each has in real time. (That data powers this very nifty map that lets you see the number of bikes at each station over the previous 24 hours.) Combined with the fact that my phone knows where I am on the map, it makes it very easy to use.
More broadly, this also contributes to a sense of having lots of options available for getting around the city. To get to any given destination, I can walk, take Metro or a bus, hail a taxi, or grab a CaBi bicycle. I can even take a Zipcar on a few minutes' notice, though that needs to be returned to the same location. The usability of all these forms of transportation (except taxis) is greatly improved by a smartphone, which, in addition to pointing me to CaBi bicycles, can also tell me which way I'm walking, the arrival time of the next bus, or the location of the nearest available Zipcar.