Synergy is just too cool. This is the kind of software that should just be implemented onto evey networked computer by default. It gives you the ability to have control over as many networked computers as necessary using only one keyboard and one mouse. Of course, you need a monitor on every computer that will be used, as it uses 'screens'.
First off, the main purpose that I see this being handy with is server/client or server/server installations. Using a client on the network, you can control your server graphically without the need for using another keyboard and mouse. This could also be handy in server rooms when you need to run various cron jobs on varying servers. Again, one mouse and one keyboard. Lastly, I see this being useful in development environments, where cross platform programs are being built and tested. The dev only needs, that's right, one keyboard and one mouse.
I think this would go without saying, but it helps if the monitors are right next to each other.
I have this installed at home. My bed-ridden laptop, Hercules, is the main computer for most of my work. However, it conveniently sits right next to my server hosting this site and others. The two monitors are only inches apart from each other. Now, rather than always pulling out the drawer where the server keyboard and mouse reside, I can just use the mouse and keyboard connected to the laptop (I prefer the tactile response of the keyboard on my laptop to that of the one connected to the server). Controlling the server is as easy as just moving the mouse past the edge of the screen and onto the screen of the server. Getting the mouse back to my laptop is as easy as moving the mouse back past the former screen edge.
All of this is done over the network, obviously. So in comes the question about security. No worries, if security is what you need, SSH can handle synergy just fine. Also, screensavers can be synchronized together, and locking both screens can only require one user/pass combination. Synergy also understands multiple monitors on a single computer as well as those connected on the network. All-in-all, it's a one-stop solution for multiple computer manipulation.
Best of all? Synergy is OSS and licensed under the GPL. It is available in the Ubuntu repositories: 'sudo apt-get install synergy', and can work on multiple platforms, inlcuding Linux, Mac OS X and even Windows. Trust me. If you have two monitors right next to each other connected to seperate computers, you'll wonder how you ever lived without it.