An application we're developing has a good advertising opportunity: people who view a certain page we publish are almost certainly going to want to make very specific (corresponding) travel reservations. We have a pretty good guess of the dates of arrival and departure, and an even better guess of the destination airport. We can thus, as a first approximation, pre-fill most of that information on their behalf. (Contrast this to travel sites such as the Continental Airlines ticket purchase interface: after you purchase a ticket it asks whether you'd like a room or car; at least as of recently, if you clicked on either of these, you had to enter your data all over again, despite having just purchased a ticket.)
It appears you can use ITA Software's Matrix with pre-filling: most of the useful fields are exposed. Sadly, all this does is search, because the interface doesn't let you actually buy a ticket.
More importantly, in return for referring customers, we'd love to get a referral kickback, just like Amazon.com's Associates program. And from what I can tell (on the authority of some people in the know), there doesn't appear to be any such facility at all.
It seems like a pretty good service that benefits all three parties (the site users, the travel sites, and us), and yet none of the major travel sites seem to offer such a program. (The person of authority I spoke with suggested it's because their margins are piss-poor, though it's not then clear how they stay in business at all.)