Google to launch flight search service soon
venturebeat.com
venturebeat.com
Kayak used to have a free API, but they shut it down "because of constly misuse". I'm not aware of any other service that's targeted at individual developers - most of these APIs are intended for huge companies with correspondingly deep pockets.
It'd be cool if they could handle other itinerary and reservation tasks. Though I don't know how they'd necessarily make money like that.
Man Google has it covered. I'm trying hard to think of an online space they're not in.
(downvote away)