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Over at O'Reilly's Mac DevCenter, Erica Sadun points out a little-known (at least to me) feature in the current version of iChat:

By control-clicking (right-clicking) a buddy's name, a contextual menu pops up offering the option to invite that person to a one-way video chat. This means that they can watch your video but will not send video back to you in return.

For example, crafts like knitting are notoriously hard to teach without visual cues, and even the most detailed books often suffer from their lack of moving pictures. When I learned to knit, I used books as a crutch, but actually learning required sitting next to my mother-in-law to see how it was done. Even now, when I hit a roadblock with a new technique, I need to wait till the next time we're together. But if we had a feature like this, we likely wouldn't have to wait. I could just "look over her shoulder" as she described what she was doing.

The most obvious benefit of one-way video is that it doesn't require the recipient to have a video camera on their end, but as Erica notes, for instructional content as with streaming movies, one-way video has another notable advantage over two-way video conferencing:

you don't have to watch the other person watching your video. You don't have to see them adjusting their hair, performing nasal maintenance, or any of the other unconscious things people do when they get involved in watching TV as opposed to engaging actively in a social situation.