Synergy is a application that allows you to share one mouse and keyboard with other PCs, similar to a KVM switch. Typically a person will have a laptop and one or more desktops at his desk - so instead of keeping a separate keyboard/mouse for each, you can just use one to control all of them.
I use synergy running on my laptop (Windows 7) to control my desktop (Ubuntu 13.04) - all with the built-in mouse an keyboard on the laptop.
They launch at startup. However on Ubuntu, I created an entry in Startup Applications. However, this will only launch once the user is logged in. To launch Synergy earlier, you can use this recipe (I have not tested it yet).
Update 2017: since using a Macbook, and Hackintosh on my desktop, I use the native Screensharing to control the hackintosh