Windows/Mac/Linux: Popcorn Time is a free open-source tool for browsing, downloading and watching movies. Browse the app's huge collection of movies and documentaries, pick one, and click play. The app torrents the movie, starts playing it immediately, and streams it while the download finishes in the background.
Popcorn Time's best features are its interface and its media library. Other tools, most notably µTorrent, our favourite for Windows and for Mac can also stream a torrent as it's downloading in the background. This app makes it seem like it's just playing though -- click "Watch Now" and a video will "buffer" before the download starts and the video starts playing. Once you have the video, you can watch it again anytime without re-downloading the movie.
The library is broken into genres on the left sidebar, including everything from animation, sci-fi and fantasy to family, sport and westerns. Click on any of them to see poster cards for each available film, or just search for what you're interested in using the keyword search bar at the top. I searched for a few not-so-obscure titles and didn't turn up anything, so your mileage may vary there.
Overall, Popcorn Time is as simple to use as XBMC. Since the app is available for virtually every desktop platform, it makes a great addition to a home theatre PC.
The downside? Many of the movies in Popcorn Time are copyrighted, though there's plenty of public domain and otherwise free content to watch. As with any tool of this nature, how you use it determines its legality, and the folks behind Popcorn Time pack it full of warnings. They also explain that they're a gateway only -- they don't host anything (not even the torrents), there are no ads, they don't make any money from the project, and it's strictly a labour of love. As always, it's up to you how you use your technology. Hit the link below to read more or give it a try.
Update: Popcorn Time is having trouble with its download links, so if you have trouble with the link below, you can download the app at its GitHub repository here, or grab the Mac version here or the Windows version here (for as long as those links last.)