The National Library of Australia has made a version of its union catalog openly available on the web (an enhanced version is available to subscribers).
Libraries Australia lets you discover what's in Australian libraries. You can find it, borrow it, copy it or buy it. [Libraries Australia - Home]This is another example of how a large bibliographic resource is being used to support the discover-locate-request-deliver chain on the open web. I have discussed some other examples before. It is moving discovery2delivery to the 'network level': in long tail terms, it is aggregating supply (providing a more unified and complete service experience over a distributed collection). It will be interesting to see to what extent it aggregates demand: will it develop strong gravitational attraction for users? Will it increase use of library collections?
The strength of having a union catalog in this case is that it records relationships between found items and holding libraries. The value is as much in the connections it can make as in the bibliographic data itself. There is also quite a bit of related infrastructure here: the enviable Australian Libraries Gateway, links directly into library OPACs, integration with a variety of fulfilment options, and so on.