Microsoft launched a developer preview for Windows 8 a while back. But now they have just released it for anyone and everyone. The file size can vary - 2.5GB to 3.3GB - depending on your version of Windows so make sure you're on WiFi and have a fast connection or you'll be waiting a while.
With this consumer preview you'll get the beautiful and minimalistic Metro styling which Microsoft are adding to everything. You'll also get access to the apps available for Windows 8. Requirements include at least a 1Ghz processor, 1GB of RAM and 16GB of free storage space.
All apps available for Windows 8 will be available for free during this preview.
I have yet to try it, but if you've been able to install it through Parallels on your Mac, let me know in the comments below!