Have you ever wanted an xml document that contained the entire set of species for a paticular kingdom? The Integrated Taxonomic Information System is the site for you.
The goal is to create an easily accessible database with reliable information on species names and their hierarchical classification. The database will be reviewed periodically to ensure high quality with valid classifications, revisions, and additions of newly described species. The ITIS includes documented taxonomic information of flora and fauna from both aquatic and terrestrial habitats.
Excellent use of the web to deliver data to the research community. What they don’t have yet (as far as I could tell) is an RSS feed listing new species as they are discovered. That would be extra nifty.