Nimi Visuals (free) adds a number of special effects to your Windows desktop, such as transparent windows or glitzy lighting effects when you close a window.
Using Nimi couldn't be simpler. You do need to have Microsoft .NET 3.5 installed before you run it (or you'll get an uninformative error), but you otherwise don't even need to install the small app. Just save the file where you want it, and then double-click it to tuck it away in your system tray.
You can then right-click its icon to have Nimi start with Windows, and then pick and choose from 33 built-in effects. You might get some slight productivity benefit from some effects, such as one that allows you to get to your desktop icons without having to minimize all your windows, but the effects are really all about eye candy.
And although fun, Nimi is clearly a work in progress. The author emphasizes that as a side project, the program may have some rough edges. In my testing, I found that some effects, such as the "Invisible Desktop" option that should hide the clutter of icons and shortcut until you click on the desktop, didn't seem to work.
Also, while Nimi can quickly and easily provide some fun special effects, it isn't meant to provide a full desktop makeover. For that, try a broader customization tool like WindowBlinds.