Is there a reliable, automatic subtitle downloader for Mac OS X? On Ubuntu I use SMPlayer or SubDownloader, which both reliably retrieve subtitles from opensubtitles.org. SMplayer even does this automatically for the file you are currently viewing.

So far I've tried SolEol, FileBot, SubDownloader (which is a bit behind on versions for Mac OS; seems like the API URL for opensubtitles.org changed) and Undertext. None of them work reliably, some of them not at all.
I know this is an old question, but I was searching for this same answer, and found a great solution. 

Subliminal will allow you to set up a cron job, (or manually enter from CLI) whole directory trees, and download subtitles. You can also set it up as a post processor for SABNzbd or SickBeard or CouchPotato.

It's working great for me.