Open Source Initiative
What is it?
As stated on their website:
We are the stewards of the Open Source Definition (OSD) and the community-recognized body for reviewing and approving licenses as OSD-conformant.
How to support
The Open Source Initiative takes donations in several ways, including Flattr. You can also join as a member. For people who pay taxes in the US, it is of interest that OSI is a 501(c)(3) deductible organization.
The OSI supports open source in so many different ways that it's hard to summarize: They educate, promote and support open source and open source projects.