Mac OS X 10.4 Tiger
So I've been using the newest OS X released from Apple in April and it's simply amazing. It never ceases to amaze me how Apple makes just the right tweaks and fixes and keeps improving the UI and overall user experience. You can check out all the new features here: Mac OS X Tiger
If you're not a Mac user then I highly suggest you make the switch now. It's true what you've been hearing. They're easier to use (if that's your thing) and powerful (if you want to configure your OS). They're beautiful, stable, long-lasting and yes, virus-free. Take it or leave it, but I'm getting my family to switch with their next purchase!
That's all the shit in my head right now.