How can I control multiple computers easily?

There are several software packages that provide the ability to use a single keyboard and mouse to control multiple computers ranging from free to low cost.

Microsoft's Mouse Without Borders support

Mouse Without Borders is a free utility for Windows developed by employees at Microsoft. Additionally, it is the simplest of the four to setup and configure since it doesn't require any knowledge of computer networking to get started.

Multiplicity

Multiplicity by Object Desktop is a low-cost easy to install and configure keyboard/mouse sharing utility that works across several versions of Windows.

Synergy

Synergy is an open-source (free) application that is a little more difficult to install and configure; however, works across Windows, Macintosh and Linux.

InputDirector

InputDirector is a free application that is very similar to Multiplicity; however, it is not as easy to setup or configure... but free.