Live Wallpaper

I once saw my brother's Motorola Droid X phone with a home screen that had a aquarium background that acted like an aquarium screensaver I have. For smart phones, it was the coolest thing I had seen. At the time I didn't have an iPhone but now that I do, I have been searching for what he had to see if I could add it to my iPhone 4s. I asked him which app he had that did that and he mentioned it was a "live wallpaper". Maybe this is just a feature ability for Droid OS but I figure who better to ask than the Apple community! So, have any of you found or used what I am referring to?

To better help visulize what I'm talking about, picture one of those aquarium screensavers in the background of a home screen with the apps and such in front.