Jeremy Keith has developed a web app called Adactio Elsewhere, which uses APIs to collect scattered pieces of Web content into one place. It's a nice idea and one that Erik Benson was exploring not so long ago. Erik ended up piping all his content pieces through to his weblog via the Bloglines API.
Jeremy built his app using Ajax (what else) and currently he's got Flickr, del.icio.us, Amazon and Upcoming in there. He's also included an RSS Reader, which runs off his OPML file - although I wonder whether he can use the Bloglines API for this, for added functionality? It would also be great to see 43Things added, as it has an API.
With sites like 43Things and Yahoo360 (and Flickr for that matter), I simply don't have time to visit them as much as I'd like to. Having everything in one place would potentially make me use services like 43Things and Yahoo360 more often. Another reason why adding an API is almost essential for social software services these days.