YouTube recently introduced a beta feature in YouTube: video annotation. You can add notes, links, highlight specific areas within a video and more. Quite nifty, if you ask me.
Annotation in YouTube is limited to the video uploader. There used to be a collaborative video annotation service, Mojiti. Unfortunately, this service, which was previously featured here, has shut down.
Another related service is dotSUB. This one allows people to collaboratively add subtitles to videos. Terrific for educators, especially those in language education.
Via Digital-Lifestyles, Hat tip to Vantan, who highlighted this in her Google Reader shared items.