Second Life’s Strawberry Singh has an article on Avatar Physics (booby bounce) with mesh bodies: Second Life Avatar Physics. It includes a link to this video: (dancing starts about halfway in)

Strawberry’s berries bounce…

Strawberry points us to Firestorm’s tutorial on Avatar Physics, which teaches you how to make your own physics layers. She has made her own layers for the Maitreya Lara mesh body. These are almost free (L$1).