Some of these aren't necessarily Mac-specific.... but I thought I'd share some of my favorite Mac tools:
iTerm - I've been holding out on iTerm for a while. First it was the keyboard delays, then it was the lack of good anti-aliasing, then it was problems with terminal emulation. All my gripes have apparently been fixed. I'm loving it like I loved Terminal.app. Now I just have a lot less clutter.
CoRD - With as many Windows servers as I have to administrate, I have no idea how I ever lived without this. Microsoft's RDP client doesn't hold a candle to CoRD.
MacFUSE - Nothing like being able to mount remote filesystems over SSH...
MacFusion -...and having a nice GUI to configure it all with.
SimplifyMedia - Painless sharing of iTunes music with people over the Internet. I use it to stream music from home to work, but you can also use it to share music with some of your friends. No network configuration needed. It's incredible.
That's all I got for now. What are some of your favorite tools? Please avoid starting any editor wars!