Universal Subtitles (or Amara) is a free online tool that allows you to subtitle exiting video sources (you can enter the URL of an Ogg, WebM, flv, mp4, Youtube, Vimeo or Dailymotion video) in many different languages. If you have ever tried to construct a .srt file from hand, you know how difficult and time consuming this process can be. Amara’s GUI (graphical user interface) makes subtitling videos pretty painless. First, you watch the video in short 8-second clips (you can adjust this as needed) and type what you hear. Then, you watch the video in real-time and press the down arrow key to add the subtitle text that you have already entered at the appropriate time. Lastly, you do some fine tune adjustments to make sure the text displays at the exact times that you want it to.
Amara was meant to be a collaborative effort. This means that other users can edit the subtitles you have added (presumably improving them) or add additional subtitles to your video in other languages. You might want to download the subtitles file you have created once you are finished for your own safe keeping.
Check it out here: http://www.universalsubtitles.org/en/