synergy

synergy: [noun] a mutually advantageous conjunction of distinct elements

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).

Redirecting the mouse and keyboard is as simple as moving the mouse off the edge of your screen. Synergy also merges the clipboards of all the systems into one, allowing cut-and-paste between systems. Furthermore, it synchronizes screen savers so they all start and stop together and, if screen locking is enabled, only one screen requires a password to unlock them all. Learn more about how it works.

Last release was version 1.3.1 on April 2, 2006

Mostly stable, although it seems like it has a few bugs... It could definitely do with daemon scripts.

EDIT: Chris Schoeneman (creator/dev) contacted me after mailing him and says he's working on 2.0 but a pre-release isn't ready yet.

Someone should hire this guy to do this full time, there is no other project remotely like it to my knowledge. Unless you say thing like xinerama or a kvm are like it (I don't think those solve the same problem).