An explanation of open data, why it's important and how you can do it yourself.
California Open Data Handbook: This is a guide published by the Stewards of Change Institute that explains what open data is, why it's important and the technical nuances behind opening it up.
Sunlight Foundation, Open Data Guidelines: The Sunlight Foundation is a well-known open data advocate. These guidelines offer advice and best practices for governments that want to start an open data project.
Open Data Institute: In Europe and across the globe the ODI is making waves by linking open data with businesses and organizations. The organization offers tools, tips and classes on open data use in addition to certification of open data types.
The Data Transparency Coalition: A transparency lobbying group that has been working with legislators in Washington D.C. and has a website explaining and monitoring the issues around open data.
Open Data Definition: Want examples of "open" licenses that can be added to your data? This site has a collection of licenses for reference and use.