English Short title Catalogue (ESTC)

The British Library has made the ESTC available online. The ESTC lists over 460,000 items held in the British Library and over 2,000 other libraries, that were published between 1473 and 1800. Items are mainly (but not exclusively) in English. This is still very much a work in progress so new records are being added all the time.
http://estc.bl.uk