The Open Knowledge Foundation summarizes "Open Data" as
A piece of data is open if anyone is free to use, reuse, and redistribute it — subject only, at most, to the requirement to attribute and/or share-alike.More specifically, the open data movement aims to make data (e.g. scientific dataset) freely available to people and organizations to re-use and republish as they wish through Open Access licenses. Note that it recommended to include any additional data generated from analysis, etc. as part of the re-published dataset.
Over the last few years, government and scientific institutions have published a wide range of datasets. The data.gov website published a list of 294 sites across 50 countries, including the United States and Belgium, covering domains, such as science, government and economics. Ben Jones has created a tool to easily navigate across the different sites.