One of the exclusive features of Windows Vista Ultimate is Dreamscene which enables you to display animated backgrounds on your operating system.
I never liked the feature much, not only because it may add distraction to the desktop which may make influence how you work on the system, but also since it requires more system resources than a plain background image, and because the default videos that Dreamscene ships with are not really that impressive after all.
That was however before I discovered user generated Dreamscene backgrounds that blew my mind. Those backgrounds are eye friendly, they show passing clouds, ripples and rain drops. Lots of animations that relax the eye and are pleasing at the same time.
Dreamscene users will have to download the free Descapes software to add the following Dreamscene backgrounds to their Vista Ultimate system.
You can find many more backgrounds at the Wincustomize Dream Gallery. Please note that you may also use the backgrounds on other versions of Vista or Windows if you have enabled the video wallpaper feature on them.
The site itself hosts a collection of more than 1000 different Dreamscene backgrounds that you can download to your system.
You can use filters displayed at the top to change the sorting algorithm, use the search to find specifically themed backgrounds, or browse the popular or featured listings instead which contain some of the most popular backgrounds the site has to offer.
Downloads are free and depending on the Dreamscene video you download may take a while to download to your system. Many of the videos offered here are 20 to 40 Megabyte in size, and if you plan to download a couple of them, you best use a download manager to do so.
Update: Last time I checked, Win Customize listed almost 3000 Dreamscene videos for you to download and use with Stardock's DeskScapes software.
Note that Dreamscene has not found its way officially to Windows 7 or newer versions of Windows, but that you can enable DreamScene on Windows 7 if you want.