IUCN Red List of Threatened Species
The Red List provides a searchable database describing the taxonomic, conservation status and distribution information primarily for Birds and Mammals but the IUCN state:
"Work is underway to re-assess all mammals (approximately 6,000 species) and all birds (approximately 10,000 species); and assess amphibians (approximately 5,000 species); reptiles (approximately 8,000 species); freshwater fish (approximately 10,000 species), sharks, rays and chimaeras (approximately 1,000 species) and freshwater molluscs (approximately 5,000 species). Plant, invertebrate and marine species assessments are also in progress. By 2008 it is hoped that a worldwide biodiversity assessment will be possible using a representative sample of taxa."
This site also provide summery statistics for threaten species showing total numbers and summery statistics for various threatened status groups. The database also lists extinct species.
An valuable resource within a well developed site. Information is well laid out with good supporting information. A must bookmark for anyone serious on conservation.