One thing that has been annoying me for ages is that no video player on the web allows you to write comments for a specific time in the video that get displayed as plain text. Viddler allows you to comment at a certain time and it appears in the video, but the benefits of time based captioning both in terms of accessibility and SEO didn’t quite transpire to any video site maintainers yet.
Start the video and hit the pause button to add a new caption. You can delete captions by hitting the x links and you can jump back to the section of the video by clicking the time stamp.
Check the source for how it is done. In order to make this a service, all you need to do is have a backend script that gets all the form fields and store it in a DB.