TripIt, which aggregates people’s travel arrangements from various web sites into one web itinerary, is rolling out a referral program today that lets developers who build apps on top of its API receive a 10-20 percent cut of the sales from its TripIt Pro premium subscription service. The San Francisco-based startup released TripIt Pro, which offers customers real-time flight information for $69 a year, in June.

More than 250 developers are using TripIt’s API to build web and mobile apps, according to founder and President Gregg Brockway. Though the company offers a free iPhone application, its API is also used by developers on other mobile platforms and travel applications, and by online address book and social network service Plaxo. TripIt also developed an application for LinkedIn last year.

Separately, TripIt is partnering with travel web site Hotwire, which Brockway co-founded before leaving some five years ago. The deal will let Hotwire users send all their travel information to TripIt, which will then create a free web travel itinerary for them.

The startup pulled in $5.1 million in Series B funding last spring from Sabre Holdings, O’Reilly AlphaTech Ventures and European Founders Fund, bringing the total amount raised to $6.1 million.