metaweb.com and freebase.com
"Freebase is a vast, free, open online database of structured knowledge" - from their web site.
One interesting thing, besides the interesting technology for storing and querying structured data where both the user can define her own categories and use system wide categories, is that the content that hosted is freely licensed under Creative Commons, GNU documentation license, or in the public domain.
You need to request an invitation, and then the documentation provides information on accessing Freebase. I experimented during lunch time with their Python client APIs - cool stuff.