Thanks to a post on The Federated Search Blog linking to this page: Articles on Discovery, part of a Google Sites "wiki" called Unified Resource Discovery Comparison. Definitely a well-rounded resource, and one I've just requested write-access to.
Missing from the list is a pre-print by Jason Vaughan called Investigations into Library Web Scale Discovery Services (PDF), to be published in an upcoming issue of Information Technology and Libraries. Get it while you can, as the site mentions the preprint will be removed once it hits paper.
