Synergy is one of my favourite applications for my home and work computers. It 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 also has the added bonus of being able to share the clipboard between computers.
Ok so this is not a piece of software that is much use to most people as it is designed for users with multiple computers on a desk, but I find it better than many other solutions I have tried out such as KVMs and remote control applications.
In the office I have my main work PC and also the work server which stores all the websites etc on it and therefore being able to easily switch over to the server using the same keyboard saves a lot of time and space.
At home, I find this application even more use, I have my PC for normal web use and gaming etc, but in the same room I have a media PC/server connected to my big screen. It is a great tool for starting movies or setting up downloads on my server whilst not having to move far from my desk.
One of the main concerns you should be aware of when using Synergy is It does not encrypt its traffic on its own. If you use it in a trusted network everything is fine, but if you want to use synergy anywhere else and do not like passwords sent via plain text over the network you should use ssh-encryption.