Travel itinerary tool TripIt has opened an API that enables outside developers to incorporate the tool into other services — including business travel expense trackers, flight trackers and travel social networks.

TripIt enables users to keep track of travel itineraries in one location. Users can enter itineraries manually from the website, or forward airline itineraries to plans [at] tripit.com.

Business travelers using Expensd to track transportation and hotel costs can now auto-import TripIt itineraries into their expense reports. iPhone users with FlightTrack can see if their flights, logged via TripIt, are on time; and Facebook app Where I've Been will enable users to access past visits through TripIt, then plot them on maps in their profiles.

Prior to the API announcement, LinkedIn added My Travel by Tripit to its list of features in November 2008. The service enables colleagues traveling to the same city to locate each other.

San Francisco-based TripIt started off with $1 million from O’Reilly AlphaTech Ventures and has raised some $6.1 million to date. It launched at the TechCrunch40 in Oct. '07.

TechCrunch blogger Michael Arrington included it in his 2008 and 2009 lists of "Products I Can't Live Without," citing its simplicity and ease of use.

The next step, he says, would be for major travel booking sites to "get a clue" and put "add to TripIt" buttons on their homepages, making it unnecessary for users to forward airline confirmations to their TripIt accounts.