Mashable.com has compiled a truly extensive list of free, open source applications with links to downloads and/or developer sites. While the list is hardly focused on education, nor is it exhaustive, it's a great place to start for schools looking to save money on licensing costs and start using open source software. It even includes quite a bit of Windows software, so this isn't just for budding Linux geeks.
Want to cover personal or small business finance with an accounting or real-world math class? Check out their 12 financial apps. Graphics design without Adobe or Autocad? You bet - try any of the 44 listed programs. Care to teach customer relationship management (CRM) in those business and marketing classes? This site lists 18 CRM applications.
The usual productivity suspects are here as well and it serves as a nice who's who of open source.