If you have multiple computers, each with it’s own monitor then it can get really confusing having multiple keyboards and mice in front of you, not to mention somewhat disrupting to your workflow every time you have to change between them and reposition your hands appropriately.
Having a KVM switch can help with this, but only lets you use a single monitor at once.
There is a great bit of software for Windows and Linux called Synergy2 that lets you share the keyboard and mouse from one computer between multiple machines. Synergy does have a Mac OS X port, but in the author’s own words it is “incomplete”.
Fortunately, there is a great bit of donation-ware software for Mac OS X called “Teleport”. This essentially does the same thing as Synergy, but in a nice easy to use Mac preference pane with full zeroconf auto-discovery of other Teleport equipped Macs, certificates for authentication and the option of encryption. It can also synchronise the clipboard between two Macs.
Take a look at http://abyssoft.com/software/teleport/ to download it, and if you like it as much as me then I would encourage you to donate.