CKAN software provides a hub for datasets. The flagship site running CKAN is theDataHub.org but it is also used for dozens of other open data websites run by governments, agencies and citizens.
CKAN provides a place to search for open knowledge resources as well as register your own - be that a set of Shakespeare's works, a global population density database, the voting records of MPs, or 30 years of US patents.
CKAN is an abbreviation for 'Comprehensive Knowledge Archive Network'. Those familiar with Freshmeat or CPAN can think of CKAN as providing an analogous service for open data and knowledge.