Synergy is a system that allows you to control multiple computers (with multiple screens) with a single mouse and keyboard. It also supports clipboard sharing.
To install it, though, for Linux or Mac, all you had was command line. QSynergy gives you a GUI to set it up on Linux, Mac and Windows.
Synergy itself hasn’t been updated in a few years (since 2006, though it still works under windows 7), and since then a fork has come up called Synergy+ which has dived into the bugs and is an active project.