Synergy is an open-source application for Windows, Mac OS, and Unix/Linux/Solaris, which allows you to share a single keyboard and mouse across multiple computers with multiple monitors. So, for example, if you have a PC, a Mac, and a Linux box sitting next to each other on your desk, each with its own monitor, you can use a single keyboard/mouse to control them all. Moving the mouse to the edge of one screen causes the cursor to jump to the next; the keyboard/mouse control whichever computer the cursor is currently on. I'm not sure if I'm clearly describing how this works; check
this page for the author's explanation.