Hi loom001, I've had my first Macintosh in a long time since Christmas. Prior to that I spent most of my non-working hours running linux, I think I can talk a bit about Open Source software under MacOS X.
MacOS X has really opened up the potential to use Open Source software with Apple products partially because it's underpinnings are UNIX which is where most of the Open Source software is developed.
The first site I would recommend would be <a href=http://fink.sourceforge.net/>fink</a>. Fink is an Open Source distribution that runs on top of MacOS X. You install a package manager and then select what software you want installed. You can install gimp for image editing, various games and even scientific applications. You didn't say what you wanted so I'm being very vague.
The second site I'll point you at is at Apple itself, and that's their <a href=http://www.apple.com/macosx/x11/>X11</a> implementation since most Open Source software with any sort of gui runs under X11.
For freeware there's a wide variety available and the good thing is that most of it is native Aqua, so it has an identical look and feel to any other MacOS X application. I usually check out <a href=http://www.macosxapps.com>MacOSX Apps</a>.