Jon Stahl points us to the Nonprofit Open Source Initiative (NOSI) release of their big "think piece," "Choosing and Using Open Source Software: A Primer for Nonprofits" (pdf). The Primer covers the reasons why non-profit organizations should choose open source and free software over proprietary applications, and goes over a variety of selection and implementation scenarios. It's reasonably well-done, although it does have some minor flaws, as Jon points out. Rather than simply repeat his observations, I encourage you to download the Primer then go read Jon's detailed entry.
It's a good article, and one that seems to have been well received on many lists related to ICT and the Digital Divide. At least the ones I've been on.