Karlsruher Virtual Catalogue (KVK)
a meta catalogue. The search options spread over german union catalogues to national libraries of the world.

ZACK!
is a WWW-Gateway for Z39.50-Servers and also a meta catalogue for german and international online catalogues (to switch to german sources, klick [de] on the left). Duplicates are recognized; and a short list of matches without double entries is offered.

WorldCat is a global union catalogue of books, articles and media. Option of personalization (sorting results in thematical lists and tagging).

Dandelon is a enriched catalogue with table of contents in PDF.