Microsoft opened up Windows 8 Developer downloads some time ago. Developers (or even you) can download a copy of the still-in-development Windows 8 for testing purposes and to develop Metro-style applications for it for free from Microsoft directly.
Seems like Microsoft will call the next version of its flagship OS "Windows 8". Windows 8 Developer Preview is available in 32- or 64-bit and with or without "developer tools" as an .ISO file. It can be put onto a flash drive or burned to a CD to be installed on a computer. The version with the developer tools can be installed as a fresh OS only, while the one without it can be installed as an 'upgrade', preserving user settings, files and programs. However, it is recommended to install Windows 8 on a second computer, since it is a very, very early preview of the next-generation Windows.
The downloads vary in size from 2.8 to 4.8 GB depending on the version you pick. System requirements, download links and all other information is available on Microsoft's download page.
Some Reddit users put together guides to install Windows 8 (WARNING: geeky language) on a virtual machine (using VirtualBox) and without bootable media on another partition. Someone also found a torrent with the same download available. The poster says that the torrent has Microsoft's download server, which means that the torrent may be legit. Here's a guide by Scott Hanselman that shows you how to install it on a virtual hard drive (VHD) using USB media.
WARNING: This version of Windows is to be installed at your own risk, as it is a very primitive version and may contain many bugs, big security holes and other problems. This release may be a lot different from the final release. Also, the developer preview will probably expire when the final version is released. It is recommended that you install this developer preview on a second machine.
Do you have anything to add or any experience to share about Windows 8? Hit the comments below! Every one of them is read.