librarything
Librarything lets you search libraries and Amazon to catalog your books, and see what people with similar collections read.
Although it doesn’t have an API to control or interact with your own collection, it does allow you to add a ‘widget’ to your webpage containing your latest entries or a random collection. The downside of this widget is that their server is sometimes slow and the widget (an url returning a JavaScript snippet document writing html) does not return xhtml. But a bit of php and some regular expressions later I managed to download the snippet once (in a while), and randomly pick items from there.