The latest version of Snowstorm 2.6.3, Linden Lab's open source Second Life viewer (which you can download here at this link) comes with an impressive innovation: Avatar body physics. Which is a very high-faluting way of saying: Virtual bouncing breasts, and an avatar booty which responds to gravitational pull. Don't take my word for it, watch this Newtonian demo video by Whirly Fizzle, which is almost certainly not safe for work viewing (unless, that is, you work in burlesque):

Breast physics, as regular Second Life users know, have been part of open source, third party Second Life viewers like Emerald for two years, contributing in significant part to their widespread adaption by the userbase. (Arguably making them more popular than the official viewer itself.) When he became Linden Lab's new CEO, Rod Humble said, of third party viewers, "I do have various features I like from each that I think we should learn from." Maybe this is what Mr. Humble had in mind? (If so, then I'm inclined to say: Rod... dude!)

In any case, if you want to try this out on your own avatar (and really, how can you not), Ms. Fizzle helpfully has a how-to after the break: