
The Open Source Community is a broad church, gaining strength from its diversity. Whilst many of us use Linux on at least one of our machines, there's a (growing) number of educators who favour Macs. The operating system upon which Macs run (OSX) is based on Unix, a system to which Linux is closely allied.
Open Source Software (OSS) for Macs is an oft-neglected topic so hopefully the recommendations below will serve to go some way to redress that imbalance! 

A wonderful, wonderful suite of programs is available for OSX called MacLibre. This features a wealth of OSS, auto-updates, and is a graduate of the Google Summer of Code competition. I would recommend any Mac user to install this - what have you got to lose?
More Open Source software for OSX is listed at Open Source Mac. A more comprehensive list is available, of course, at Sourceforge. 
Are YOU using your Mac in the classroom? We'd love to hear what OSS you're running!