Bounce is a mod pack of a set of gametypes which change the gameplay in a simple but effective way. Attacks from other players no longer deal damage. But instead reduce your mass, causing you to become lighter. The goal of Bounce is to knock your opponents into the various pools, pits, and other traps that litter the arenas. While avoiding being kicked into them yourself, in logical ways, like eating pizzas and cookies to increase your weight. . . Madness? THIS IS UN-REAL.
As a good starting point for bounce you can download the map pack. It also contains the bounce mutator. It can be downloaded at http://www.moddb.com/mods/bounce