I have come across a couple of sites recently which pass queries to library catalogs.
Primarily using Amazon.com’s Web Services API, Amazon Light 4.0 is an exercise in tying together various Web Services into a whole, adding content, context and some useful tools to the Amazon shopping/browsing experience. [Amazon Light 4.0 – About]
There is an option to find in a library using an approach based on Jon Udell’s Library Lookup.
Wikipedia has a Book Sources page that allows you to find books by ISBN in a selection of catalog sources worldwide (spotted on Science Library Pad).
We have a page of xISBN Bookmarks, each of which will take an ISBN, associate ISBNs from other versions of the same work, and look in the designated catalog. It uses our xISBN service. I thought these might have attracted more attention. Maybe we should give them a new name: maybe catalinks?