clipped from: lifehacker.com 
Boxee is a free and easy-to-use cross-platform media center that puts all your videos—and most of the web's—in one place. Loaded on a small, cheap, and sleek computer, it might just change your living room forever.
Boxee's media center gives you that, and all from one application. It's free, it's open source, it's built from the guts of the killer Xbox Media Center (which is still a quite active project itself), and it simply works. Loaded onto an Apple TV, or any TV-connected computer, Boxee also gives you free license to drop your cable or satellite dependency with hardly any regret, especially once you realize your year-to-year savings. First, let's take a closer look at what makes Boxee so great; then I'll walk you through installing Boxee on your Apple TV.