The YouTube team has just completed a major upgrade to to annotations, the interactive commentary you can add to videos. Now you can add annotations to other people's videos (with permission from the video owner, of course). And if you want to let your friends add annotations to your video, simply view the video and you will see a special URL in the "Video Owner Options" on the right (you can also access this URL from the annotations editor). You can disable the URL at any time, and you can always delete annotations added by others.
I annotated this one myself: