OEROpen educational resources (including software) are generally free or low-cost alternatives. For us, it means having access to resources that otherwise may be cost-prohibitive.
OpenOffice.org is something we have used at DePaul Catholic for several years. It is a great example of just one way we take advantage of open source software.
Another example is Stellarium. We have installed this software on every student Tablet PC for the past four years. Stellarium is planetarium software. When opened, it shows a real-time 3D sky. It also tracks satellites. If you haven't opened it, go ahead, give it a try -- but allow some time because once you open it, you'll be hooked.
OK, perhaps you have no interest in the Magi's view of the North Star, or parabolas, or satellites, or anything cosmic. Perhaps one of these open source tools is more up your alley:
Xiphos: Bible Study
Matterhorn: OpenSource software to produce lecture recordings
Opina: OpenSource survey tool
oercommons.org. But, I am forewarning you -- you cannot expect to spend five minutes here -- it is a great comprehensive site that has something for everyone.