Sharemouse is a tool that allows your mouse to float between the screens of network-connected Macs and Windows machines. However, it cannot do relative mouse movements, which rules out being able to play a 3D game on adjacent screens. The other alternatives are
synergy which looks and feels horrible compared to Sharemouse. However, it is able to do relative mouse movements. Another one is
teleport for OSX which is sort-of
70% functional and free but works only on OSX.
Sharemouse is used by some people because it has a nice carbon OSX interface which is consistent with the rest of the operating system. The menus are nice and the application feels native to the operating system. Otherwise, there is no reason to use this...