Ubuntu Linux 11.10 (Oneric Ocelot). I like using Linux because it's free and there's so many options (like, on Ubuntu you can choose using current Ubuntu, classic, or Gnome, or customize yourself if you know how... or if you don't even like Ubuntu, you can choose Fedora or another OS by Linux) and it's a community of programmers that voluntarily create and update the OS (which is constantly updating and ever changing... not to mention, when they've got a new version of Ubuntu out, the switch from the old to the new is incredibly easy!). And it's gorgeous and completely easy to use compared to Windows. I've never used a Mac, myself, but I think Ubuntu's on par with Mac. I like Mac, the only problem is that you have to purchase their computers if you want the OS. I should mention I have no problems with paying money for an OS, it's just how mega expensive it is nowadays.
In this day and age, everyone's using computers and it's literally become a necessity, so I think it's a bit ridiculous to make people pay so much for an OS that would become obsolete in a few years.