Facebook just announced a new developer platform that lets you write applications that live inside their network. You register your code with their service, and when people add your application to their profile you get a simplified markup language for your code. I don't understand all of the fine points yet, nor am I likely to any time soon, but there are sample and tutorial codes out there (in PHP5) that let me bring up a tiny "hello world" style application in under an hour.
The obvious next step is to build in some kind of interface to the library catalog, right? The simple target is something along the lines of John Blyberg's Go Go Google Gadget, which shows new and popular books in a little box. Only slightly more complicated is a search interface.
No code or examples to share yet, but if anyone wants to give this a go I'm happy to see more.