SubDownloader is an automatic movie subtitle downloader. It is a multi-platform project available for Windows, Linux and Mac OS X. It uses OpenSubtitles.org to get subtitles for your video files automatically. The first thing the application does is connects to that website. If connection cannot be made for whatever reason, the whole app is rendered useless. You seem to need to register for a user account on the website before you can use the app, too.
Once everything is set up, there are two different options to download subtitles. First, you can point the application to the file or folder containing your movie. Then it will automatically download the corresponding file, taking into consideration the name of the movie, and the release group. Secondly, you can search for a subtitle by movie name. This is less automatic, but it works well. There is another tab that allows you to quickly and easily upload subtitles to the website, so that other users can benefit from them.
In my testing, I couldn't use the feature to find subtitles automatically. Apparently, a connection to OpenSubtitles couldn't be made. I was able to search for subtitles by typing in the movie's name though.