opendatasearch.org is ... an aggregator for datasets, providing a simple and unified search interface to all of the catalogues contained. At the moment, this includes all known instances of the CKAN software, the Sunlight Foundation's National Data Catalog (and with it a large number of US-based data sources), the World Bank data catalogue, Sweden's DCat-enabled OpenGov.se and Nexedi's Data Publica portal. We've also put up search.ckan.net which provides access to the combined index of all CKANs only.
Meanwhile, Open Knowledge Foundation has released an open data manual:
This report discusses legal, social and technical aspects of open data. The manual can be used by anyone but is especially designed for those seeking to open up data. It discusses the why, what and how of open data -- why to go open, what open is, and the how to 'open' data.
Last but not least, Ryan McGreal has written a piece about pundits' debate of open public data.