I'm hunting for the right flight search api to bring flight results into Routefriend. We've been mainly testing with travelfusion, but many of the search results come up in british pounds. Cleartrip also appears to have a nice one, but with search results in rupees.
The best so far would seem to be Kayak's API, however we hit some bugs in testing. It appears the API is no longer supported...it's not linked from the Kayak labs site anymore.
Microsoft's new search engine, bing, has flight search and an API. I'm not clear yet whether the flight search results are accessible through the API. They are blocked from scraping via a robots.txt exclusion.
Anybody know of other good flight search APIs?