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! 
Well-known OSS applications also available for OSX
- Audacity (sound recorder & editor)
- Firefox (web browser)
- FreeMind (mind-mapping program)
- The GIMP (image editor)
- Handbrake (DVD ripper and video converter - formerly OSX-specific!)
- KompoZer (web page editor)
- Miro (HD video player & Bittorrent client)
- OpenOffice.org (and NeoOffice - office suites)
- VLC (versatile video player)
- XMind (mind-mapping program - can host mindmaps online)
OSS applications that are OSX-specific
- Adium (instant messaging client)
- Cyberduck (FTP client)
- ffmpegX (video transcoder)
- Q (an emulator allowing Linux, Windows and other operating systems to be run in a virtual machine)
- Quicksilver ('unified, extensible interface for working with applications, contacts, music, and other data')
- Romeo (control Mac via Bluetooth)
- Seashore (image editor)
- SimplyBurns (CD/DVD burning software)
- Transmission (Bittorrent client)
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!
Nice post, Doug.
One tool that really helps with the installation of OOS on the mac is Fink. I've used it in the past, it works like a linux repository for OOS apps for OS X.
app trap for un installation of apps if required :
http://konstochvanligasaker.se/apptrap/
Fink and X11 are often ignored by educators, but are gateways to quite a few apps. OpenOffice ran nicely on OSX under X11 quite a while ago.....
http://www.finkproject.org/doc/x11/index.php
And of course, a discussion of X11 also raises the option of using vnc (server and or client), which is handy when it comes to cross work-station remote display
For convenience sake, this link is worth a quick look see http://www.apple.com/downloads/macosx/unix_open_source/ (even a GUI for FINK!) as is this (some of the apps are mentioned above, some not) http://www.opensourcemac.org/
I was surprised that though FF was mentioned, Tbird was not. And there are version of amaya and nvu
By way of an aside, many folks aren't aware of how CUPS can make your life easier (I know of quite a few installations with thousands and thousands of students where network use of CUPS in an all Mac environment has never even been explored.)
No discussion of OS X would be compete without mentioning Darwin (http://developer.apple.com/Darwin/) and Darwin ports http://darwinports.com/
And a caveat with respect to use of the terminal, in that as with other OS in which a gui has been strapped to an OS, be aware that the OS X gui does not always effect the same files as one might twiddle if dong things at the command line and bypassing the gui can result in some intriguing results (linux users are probably familiar with the situation where they changed a conf file, but the file they changed was written over by a file employed by the gui on reboot....)
A friend installed clamXav and we freaked to discover that it was identifying PC threats! http://www.clamxav.com/ Read the docs carefully about using wth VPC and Bootcamp...
What F/OSS s/w am I running on my Macs? Well a quick flick through my Applications folder shows the following. I'll be honest and say I'm not definitively sure which are simply Free (as in free beer) and which are Free (as in free speech) so I've just listed them all...
Well what else?
As standard on OS X you have an Apache webserver, together with PHP, so you can get a MAMP stack for web development by just installing MySQL. Alternatively, you can get all this packaged together as MAMP, which is how I did the development work for this site over the Christmas holidays, together with Cyberduck and Smultron (FTP and text editing respectively)
I've also got the following installed on my MBP, which I don't think anyone else has mentioned already:
I didn't know about Q, but I do have a Ubuntu virtual machine installed, which runs on VMWare Fusion.
Miles.
Great list!
Consider adding uTorrent. It's the best torrent client out there.
http://mac.utorrent.com/
Apple Distinguished Educator Simon Elliott has an educationally focussed directory of free and open source Mac applications at TheFreeMac Classroom
You know... Transmission is part of the default install on Ubuntu. It is a great application, but it probably should not be in the OS X only category.
I use Virtualbox a lot - the licensing isn't quite as straightforward as you might like, there being a free for personal use "real" edition, and a community/GNU version - I use it to run Solaris/Ubuntu/Windows on my Mac.
Aquamacs and gVim play a big role on my Mac, as does php5, mysql, ruby on rails and other languages (perl, python, etc). I use twiki for my own notes/documentation. Oh, and openvpn too (although I use viscosity as the front end to it).
A couple more sites for OSS on OS-X:
Apple's own developer connection has quite a number of resources, which may be of interest to developers, at http://developer.apple.com/opensource/index.html, including webkit (on which Safari is built), and instant networking wizardry Bonjour.
More accessible is FreeSMUG, the Free open source Software Mac User Group, at http://www.freesmug.org/.