National capitals seem to have a way of attracting carsharing companies: London, Berlin, Paris, Madrid and Amsterdam all have several competing carshare operators. With the demise of Flexcar in late 2007, Zipcar has had Washington DC mostly to itself.
Now both Hertz On Demand and car2go have announced a full-scale operations. Hertz On Demand publicly launching the city and car2go intending to launch later this year. This will be an interesting year in DC!

With the return of competition to Washington, Zipcar has competition in all four of what it calls its "established markets" — Boston, New York, San Francisco and Washington, D.C. These cities were all launched between 2000-2005. (That these markets are their "established" markets always seemed a bit arbitrary to me, since they now operate several markets established by Flexcar of the same vintage, and which were launched with similar level of investment - Seattle (2000) and Portland (1998).
DC's thoughtful online new source TBD Online thinks Hertz and car2go may be "better deals" than Zipcar, primarily because of no annual fees, but Zipcar has shown it will match and beat the deals of competitors in other markets, so I fully expect them to effectively counter with equally attractive new member packages, especially when the weather gets better and the spring marketing swings into action. And pricing for Hertz and Zip are essentially the same and car2go's pricing really isn't comparable.

Hertz has indicated they would NOT be offering one-way service, at least not initially, as they do in New York City. But, my guess is they will do start offering one-way to the airports sooner rather than later. Although offering one-way to airports is great for marketing and a wonderful service for customers, it's probably closer to self-drive taxi than anything else. In the meantime, their vehicles have the clunky but useful Never Lost navigation system and their insurance has a $250 deductible rather than $750 for Zipcar.

I'm still waiting to see what sort of impact car2go's unique one-way service only with Smart cars has on traditional "round-trip" carsharing services. My suspicion that the small 2-seat vehicle defines a different type of trip than that of "round trip" carsharing with its bigger 4-5 passenger vehicles, is now somewhat confirmed in other car2go cities. Fortunately, an early study from Europe is showing that vehicle ownership rates for car2go members is lower after joining, though not as much as traditional round-trip carsharing services.
Some Other Perspectives on Competition
Is competition bad or good for carsharing? It sort of depends on who you're asking. It's probably good for the companies, since multiple players in the market likely provides some legitimacy to this new mobility option in the minds of some prospective customers. But as far as the allocation of public resources, such as on-street parking spaces, it may be a mixed bag since most customers are only members of one service, at least of the round-trip type (I fully expect many Zipcar and Hertz On Demand members will also have car2go membership.)
For what it's worth: during the competition deliberations when Zipcar was acquiring Streetcar in London at least one city (Islington) reported to the Competition Commission, that it preferred to keep its partnership with only a single carsharing operator since they felt it would be better for residents since they could be assured of using every carsharing in the area, and was certainly easier for them administratively.
Meanwhile, down in Raleigh, North Carolina, there are complaints that competition is hurting the growth of carsharing. Actually, I doubt it's the competition as much as the lack of commitment by the carsharing companies. WeCar has never put much juice into marketing WeCar that I've seen and the Zipcar operation there is on two college campuses and, in general, they seem to keep these operations close to campus and not use them as springboards to developing city-wide operations.