TripIt, the helpful travel site that lets you generate an itinerary by simply forwarding the service your Email confirmations from hotels and airlines, has opened up an API for outside developers. The API will give third party applications access to TripIt’s itinerary sysytem, which now accepts data from 350 travel sites. Developers can find all the details for joining the program here.

A number of applications have already used the API to implement new features. Expensd, an online service that helps business travelers manage their expenses, will use TripIt to automatically import your transit and hotel expenses. Popular iPhone application FlightTrack will use TripIt to automatically look up your flights to see if they’re on time. And Where I’ve Been, a social network application that plots your travels on a map for your friends to see, will use the API to automatically look up your destinations. Other potential uses for the API include an ‘Add To TripIt’ button that would allow travel sites to transmit your booking data to TripIt so you won’t have to forward your confirmation Email.

TripIt is a great service, especially for frequent travelers (it has been on Michael’s list of products he can’t live without for two years running). I’m excited to see what developers can come up with – hopefully some major travel booking sites will get a clue too, and start offering that ‘add to TripIt’ button so we can skip the Email forwarding.