CaptionTube is a site that lets create captions for YouTube videos. For example, you can include a transcript of the video in one or more languages and YouTube's search engine will bring more viewers.
The site lets you caption any YouTube video, not just the ones that you've uploaded, and the captions can be exported as an XML file that will be later imported at YouTube.
YouTube's blog mentions that the site has been built by John Skidgel, a user experience designer that works at Google. Another online captioning service created for YouTube is YouTube subtitler, but it's less user-friendly.
PLYMedia's research showed that subtitles increase the amount of time that users spend watching videos by about 40%. "As more and more online video is consumed in an increasing variety of settings—from office environments to noisy bars to mobile phones—it makes sense to add closed-captioning and subtitling features to digital video files. These enhancements expand opportunities for viewers to enjoy online video, even when it's impractical for them to have the sound turned on."