VideoANT is a free tool for collaboratively annotating videos. I first wrote about VideoANT back in 2010. Recently, as I learned from Nathan Hall, VideoANT received a major facelift that makes it easier to use than it was before. The first improvement to note is that the service is now built on HTML5 which means that Flash is no longer needed. The second major improvement is improved ease of annotating videos.
Using VideoANT anyone can add annotations to any publicly accessible YouTube video. To do this copy the URL of a video and paste it into the VideoANT annotation tool. Then as the video plays click the "add annotation" button when you want to add an annotation. To have others annotate the video with you, send them the VideoANT link. You are the only person that has to have a VideoANT account. Your collaborators do not need to have a VideoANT account to participate in the annotation process with you.