The WaveTube gadget for Google Wave is a YouTube player that displays the list of viewers in real time. For each participant watching the video, there is a progress bar (that is, how far into the video they have watched) that everyone else in the same wave can see.
The progress bar will remain for 24 hours by default, or for a period of time you set when first adding the gadget. WaveTube also makes it possible to drag someone else's progress bar or to change their playback status (play or pause).
You can also add annotations (notes) by clicking on the bubble next to the progress bar. An annotations will pop on top of the progress bar of the participant who has set it, at the time it was set in respect to the progression of the video.
It is also possible to insert a bigger YouTube player when setting up the gadget, up to 853x505 pixels in size and to choose the quality of the video from the player controls (up to 720p if supported).
For more info, wave eyalzh@googlewave.com.