Metro7 is a free tool which brings the Windows 8 Metro interface to your Windows 7 desktop.
And a very good recreation it is, too: colourful ties, calendar and weather widgets, Facebook notifications, a Twitter tool and so on. Clicking the Metro7 "Store" icon provides more options - Video Weather, Hotmail and Gmail tiles, a Control Panel link and more - and clicking the arrow icon top-right allows you to pin your own applications and internet shortcuts to the panel.
While this is all free, Metro7 is clearly set up to try and sell things. During setup it'll ask if it can install a trial of a commercial Registry cleaner, for instance. And more annoyingly, the default display includes tiles labelled "Online TV" and "Free PC Speed Test", which users may assume are somehow features related to Metro7, but are in fact just marketing links to a couple of websites. (Fortunately they're easily removed: right-click the tiles, select Remove and you won't be hassled again.)
Metro7 is also fairly heavy on RAM use (it had a couple of processes consuming around 180MB on our test system). And the program's a genuine beta. That is, there's no way you'll mistake it for a finished product: we found the program crashed on a regular basis.
Nice visuals, then, and if you just want an easy way to get the feel of Windows 8 on a Windows 7 system then Metro7 is worth a look. Whether you'll want to use it for long is another question.