The Windows 8 Developer Preview provides an early (but surprisingly complete) look at the next generation of Windows, complete with touch interface, brand new desktop apps, and more.
You're not a developer? You don't have a touch screen device? Don't worry, this release is aimed at developers, but anyone can use it. And while the new interface is very touch-oriented, it also works with the mouse and keyboard.
See also: Microsoft Windows 8 review
You do need to be careful where you install it, though. The Developer Preview requires a clean install, and can't be uninstalled later, so it's best to use it on a spare system, if you have one (especially as it's also an early version, buggy, incomplete, and with no support on offer from Microsoft). If you've an old laptop capable of running Windows 7, for instance, then it should be able to run Windows 8, too.
Or if you don't have the necessary hardware, then it can be safely run in a VirtualBox virtual machine, instead.
However you install the Developer Preview, once it's launched you'll find the new Metro-style desktop (if you've ever used or seen Windows Phone 7 then you'll know exactly what to expect) comes packed with extras. So as well as links to Explorer, Control Panel, IE and the old-style desktop (which is still available, if you get lost) there's a weather app, social networking tools, games, productivity apps and a whole lot more.
These are just sample applications, not what will be included with the finished Windows 8. They're still interesting to explore, though, and once you get tired of these then you can always install and run your existing apps, just as you always did. Well, that's the plan, at least - there are sure to be incompatibilities in this release, however, so don't be surprised if you have some issues.
Obligatory Microsoft notice: The Windows Developer Preview is a pre-beta version of Windows 8 for developers. These downloads include prerelease software that may change without notice. The software is provided as is, and you bear the risk of using it. It may not be stable, operate correctly or work the way the final version of the software will. It should not be used in a production environment. The features and functionality in the prerelease software may not appear in the final version. Some product features and functionality may require advanced or additional hardware, or installation of other software.