Here's a LinuxWorld list of Web 2.0 APIs that “you can really use”. APIs that are “are simply formalized interfaces to a user-centric application” — such as the Flickr API — don't count in this list. APIs in this list are supposedly for “real programming problems, either in Web applications or in desktop or server software”.
Well, while this article isn't going to dispel the perception that Linux developers have the same love/hate relationship with their clientele that drug pushers do, the APIs listed are worth looking into. They are: