Mac OS X only: Freeware program WebDesktop brings the functionality of Windows Active Desktop to the Mac… sort of.
WebDesktop places a resizable, overlaid web page on top of your Mac’s desktop wallpaper with the option to vary opacity while active/inactive. You can point WebDesktop to any web page, meaning that you can use it for fun active desktopproductivity tricks. You can set the page to refresh at intervals, so if, for example, you put your favorite productivity blog on your WebDesktop, you can have that bad boy refresh obsessively so you always stay on top of the latest content. WebDesktop is Mac OS X only freeware. — Adam Pash