Windows vs. Mac vs. Linux

It really all depends on what you want to use it for. While Linux and OS X are theoretically better gaming platforms, in practice Windows wins because practically no games run natively on Linux (and few do for OS X). It's also very versatile; it's set up so anything you might want to do can be done with minimal configuration.

OS X is great if you want something that "just works," which is (for some reason) what so many people seem to want out of Linux. It's ridiculously easy to learn and use, and the software written for it is great, if a bit limited in range.

Linux (or even BSD) is great if you have a specific purpose in mind. Got some old junk kicking around and want to make it useful? Put a minimal Linux install on it. Want to make a powerful server? Linux servers are great. Just bored and feel like screwing around? Linux is great for that, too. There's also a huge amount of free, community-written software available for most any Linux distribution, and you can also run a lot of Windows programs on Linux using WINE. And it is versatile, in that it can do most things (if with a bit of resistance).