The TripIt engineering team has built several language bindings to ease the development of client applications. The bindings all provide an interface to all of the TripIt API endpoints and data object types along with a full OAuth Consumer implementation. No additional code needs to be downloaded to begin interacting with the TripIt API.
Sign up for a developer key here: http://www.tripit.com/developer
The TripIt API support group is hosted on GitHub and can be found here: https://github.com/tripit/api/issues
Looking for the Concur Connect APIs from TripIt's parent company? Check out: https://developer.concur.com/