Here's an interesting list from the Accredited Online Universities blog which lists open source apps.
"Open Source applications are a great resource for educators not only because they’re free, but many times you can find specific applications for your students needs. Many teachers and schools have embraced the endless opportunities open source has to offer. From reducing administrative costs to promoting student and teacher interaction, instructors around the world are singing its praises. We have created a list of what we think are the best available apps out there and have categorized them into the following: Science, Language, Math, Administrative & Content Management, Interactive & Online Classrooms, Study Aids, Video & Imaging, Music, [Library &] Multimedia, Geography & History, and Mapping Tools."