Timelinely is a free service for adding annotations to YouTube videos. You can use Timelinely to add text, image, and video annotations to any public YouTube video. After you have added your annotations to a video you can share the annotated version with anyone much like you would share any other video. You can share your annotated video by embedding it into a blog post or by just giving people the link to the annotated version of the video. In the following video I demonstrate how to use Timelinely to annotate YouTube videos.
Applications for Education
One of things that I like about Timelinely is the option to include pictures and videos in your annotations. I can see the image option being used to include an alternate example for students to view when watching a math lesson. Adding a video into your annotation could be a good way to add your own commentary or clarifying comments to a video about a topic in history or current events.