Data provides the evidence for the published body of scientific knowledge, which is the foundation for all scientific progress. The more data is made openly available in a useful manner, the greater the level of transparency and reproducibility and hence the more efficient the scientific process becomes, to the benefit of society. This viewpoint is becoming mainstream among many funders, publishers, scientists, and other stakeholders in research, but barriers to achieving widespread publication of open data remain. The Open Data in Science working group at the Open Knowledge Foundation is a community that works to develop tools, applications, datasets, and guidelines to promote the open sharing of scientific data. This article focuses on the Open Knowledge Definition and the Panton Principles for Open Data in Science. We also discuss some of the tools the group has developed to facilitate the generation and use of open data and the potential uses that we hope will encourage further movement towards an open scientific knowledge commons.