I've spent way too much time on this site in the past. I found it a while ago when someone posted it on my graphic design forum. The site is called
Red Universe created by the Red Interactive Agency. It's an interactive chat where you can be different characters that dance, fly, talk or punch others. There is no need to register, but you can change your user name in the control panel on the right side.
To dance: press "home" key
To fly or walk forward: use arrow keys (holding shift makes you walk/fly faster)
To punch: use ctrl + shift
So come and play with me sometime.