Amara is a free online subtitling service.
It allows you to add or import text to create subtitles for a video hosted somewhere else on the Internet. The interface is easy to use, allowing you to play, pause and review your video as you type the text of the dialogue. You then match the text to the timing of the video. Subtitles can then be made available for other users to review and improve; or they can add extra subtitles in a different language.