Black Dragon, a third party Second Life viewer with a cult following, has a new update which comes with a sort of silly but also pretty amazing feature: MANDATORY DANCING. In other words, you can make avatars around you seem to dance (or do any other animation) at the push of some buttons. As Black Dragon creator Niran Dean explains on his blog:
How it works? Go into a crowd, go to 'Dragon - World - Animation Control' select any avatar whose animations you want to save into your list, stop, restart, freeze or slow down. Hit the the 'Save' button if you want to save them. Down below you'll then find a list of all motions you saved and who they belong to, select one you want to play and select one or more avatars in the above list, hit 'Apply To' and BAM everyone does exactly what you want them to do.
So the operative word is "seem", because the avatars aren't actually dancing -- it just seems that way on your Black Dragon viewer. Niran tells me this is an expanded update to an already existing feature: