I've been using Synergy for... what... like three years now? It's one of those integral pieces of software that I can't do without anymore. If you ever have craved a dual-screen setup for your laptop, but still want to use your desktop at the same time, Synergy is the perfect thing for you.
Synergy works like a KVM in reverse. You give it two or more machines, each with its own display. Pick the machine with the nicest keyboard and mouse - that will become the "host." You tell Synergy where the other machines' monitors are located (i.e. the laptop is to the left of the desktop's monitor) and Synergy will transmit all keyboard and mouse events to the other machines. You basically have connected your mouse and keyboard to your remote machines via TCP/IP.
For example, let's say you have a desktop at home and a laptop at work. Pretty typical setup. And you have a nice dual-monitor setup at work: you have your laptop's monitor on the left side of your desk, and a nice DVI monitor on the right side of your desk to use. When you get home, you'd like to have the same sort of setup... except you don't want to detach your desktop's monitor at home and re-attach it to your laptop every day.
Synergy will connect your laptop and your desktop together at home so one keyboard/mouse can control the contents of both screens. Copy and paste, lock screens, whatever you like. You can't drag-and-drop files from one desktop to another mind you - they're still physically separate machines. But you can verily easily browse the Web on one monitor and code in the other, all using the same keyboard.
The above image is translated from http://synergy2.sourceforge.net/about.html - but unlike the source image it isn't animated (thanks blogspot). It gives you a sense of how the desktops can sit side by side and Synergy allows the mouse cursor to "hop" over to the other screen. Do take a look at the animated version on Synergy's site to get a better sense of it.
It's cross platform, so Linux desktops and Windows desktops can still work well together. Even works with OS X. So your PowerBook can share a screen with your WinXP desktop alongside your Linux server. Nifty!