So like everyone else today who isn't on a Mac or Linux, I decided to try Google's Lively. There's a download, but it goes pretty quick.
There's no building, although the press says there will be options to create content. You can add music to a room, or link to youtube videos.
The emote system is delightfully simple; just right click on your avatar or someone else's to animate. No consent needed. So go ahead and body slam that fellow standing next to you!
The avatars are all prefab. I don't like that I can't edit my avatar by itself. Have to be in a room to do that.