I have a PC desktop and Mac laptop that share the same tiny workspace and decided to "bite the bullet" and buy a KVM to reduce clutter. The Belkin I purchased gets obstinate on occasion so, in desperation for a happier solution, I installed a project hosted on SourceForge (SF) call Synergy.
What Synergy attempts to do is replace the keyboard/mouse function of a KVM. The program passes the input device coordinates (or keyboard code) from a "server" computer to a client computer via TCP/IP. Like many SourceForge projects, installation requires dragging the application into a Terminal window. Also, the documentation and development is better on the Windows side than on the Unix/Mac side. Note that (in most cases) Mac local networking "names" end with a ".local" extension. Synergy has replaced the KVM on my desktop.
[robg adds: The Synergy homepage points out that the Mac port has some issues and lags behind the others in its development ... still, for all you new Mac mini owners out there, this might be worth a look.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20050130085923110