Microsoft has released its Windows 8 Developer Preview operating system, for PCs and tablets, to the public as a free download.
You can directly download it and try your hands on this new Windows. No additional hardware will be required for normal functionality, means if your are running Windows 7 or Windows Vista, you can also run Windows 8. Since this is Developer Preview Edition, it doesn't support upgrading from older versions. You have to install this separately.

System Requirements
•   1 gigahertz (GHz) or faster 32-bit (x86) or 64-bit (x64) processor
•   1 gigabyte (GB) RAM (32-bit) or 2 GB RAM (64-bit)
•   16 GB available hard disk space (32-bit) or 20 GB (64-bit)
•   DirectX 9 graphics device with WDDM 1.0 or higher driver
•   Taking advantage of touch input requires a screen that supports multi-touch

Microsoft has made available three different versions of Windows 8 for download (two versions built for 64-bit systems and one for 32-bit). All files are available as ISO image file.

Download links are

Windows 8 Developer Preview with Developer Tools (64-bit)
[ISO] Download Link [4.8 GB]

Windows 8 Developer Preview (64-bit)
[ISO] Download Link [3.6 GB]

Windows 8 Developer Preview (32-bit)
[ISO]Download Link [2.8 GB]

One bad thing I personally don't like that Windows 8 can't be upgraded from older versions. It will only support a fresh install. Microsoft has mentioned that this is a developer preview release and is not meant for production. It is mere a pre-beta version of Windows 8 for developers.

Microsoft has said that all Windows 7 apps should work in Windows 8 on desktops and laptops, but there are no guarantees that will hold true at this point given the OS is in development. Microsoft assured that in coming weeks the release will be updated time to time with more features and application/driver support.

The Developer Preview of Windows 8 will expire after six months. Deadline given by Microsoft is March 11th, 2012.

Suggested Readings:-
Windows Developer Preview downloads Home page
Windows 8 Developer Preview Guide (PDF)