synergy (multi-system sharing of keyboard and mouse)

Synergy lets you easily share a single mouse and keyboard between 
multiple computers with different operating systems, each with its 
own display, without special hardware. It's intended for users with 
multiple computers on their desk since each system uses its own 
monitor(s). 

Synergy is open source and released under the GNU Public License (GPL).
There is an example conf file in the docs directory