I love MacOS X. I don't mince words about it; the UI is gorgeous, simple, to-the-point, and I love it. I love Linux as well, but mainly because it's usually a challenge to get working right. After getting particularly bored the other day, and I thought to myself "You know, self, this iMac works entirely too well." Seven hours later, Gentoo is up and (mostly) running.
Major props out to the guys who wrote parted, the guy who wrote the patches to parted to let it resize HFS+ filesystems without destroying data (if I ever meet you, I owe you a beer), and whoever wrote the driver for reading HFS+ volumes under Linux. All of this stuff makes using Linux on a Mac a much more pleasant experience.
Back in OS X land, though, the guys who I'm the most impressed with are the team behind Colloquy which is hands-down, the single coolest Open Source application I've seen on OS X yet. I've seen a few other apps that I consider to have great user interfaces, but Colloquy is the only Open Source one. These guys are seriously awesome.