Short bio: Computer Scientist, FOSS supporter (read more)
Tux Machines (TM)-specific
We live in a three OS, two hardware platform world, but that's going to start changing in 2006. Apple will begin migrating Macs to Intel CPUs, and the new "Macintel" computers will be able to run Windows, Linux x86, and Mac OS X.
Windows does a lot of things right, but the underlying operating system is so full of holes it makes Swiss cheese look solid. Linux does a lot of things right, but there are still those geeky things that require you to use the terminal.
Mac OS X does almost everything right. It's so secure that not a single virus has survived in the wild. Every program written for OS X (as opposed to ported from Linux or Unix) installs easily. The underlying kernel is a bit of a hack, ported from NeXT's x86-based OS to the PowerPC, and moving the Mac to Intel CPUs will improve things.
The question is how the future will play out. Linux is already broken into many camps - KDE aficionados vs. Gnome users, Debian vs. Slackware vs. Gentoo plus dozens of other distributions. But the core of Linux is solid, powerful, efficient, and doesn't get as bogged down with multithreading as the Mac OS.
In many ways, OS X is what Linux would be with a great GUI. Likewise, Linux is in some ways what OS X could be. For all its strengths, OS X does allow tasks to so dominate the OS that everything else stops while the beach ball spins.