How can I control two computers with the same keyboard and mouse?

"There is this open-source software hosted on Sourceforge.net:
Synergy — (Git repo here)

Synergy 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's
intended for users with multiple
computers on their desk since each
system uses its own monitor(s).
Redirecting the mouse and keyboard is
as simple as moving the mouse off the
edge of your screen. Synergy also
merges the clipboards of all the
systems into one, allowing
cut-and-paste between systems.
Furthermore, it synchronizes screen
savers so they all start and stop
together and, if screen locking is
enabled, only one screen requires a
password to unlock them all.

Another free software (only for Windows):
Input Director

Input Director is a Windows
application that lets you control
multiple Windows systems using the
keyboard/mouse attached to one
computer. It is designed for folks who
have two (or more) computers set up at
home and find themselves regularly
sliding from one system to the other
(and wearing out the carpet in the
process!).
With Input Director, you
can share a single keyboard/mouse
across a set of systems. You switch
which system receives the input either
by hotkey or by moving the cursor so
that it transitions from one screen to
the other (in a very similar fashion
to a multi-monitor setup). The idea
being that you can position the
monitors from two or more systems in a
row and use a shared keyboard/mouse to
control all of them. Input Director
also supports a ""shared"" clipboard, in
which you can copy data onto the
clipboard on one system, transition
across to another and paste.
Input Director requires Windows 2000
(Service Pack 4), Windows XP (Service
Pack 2), Windows 2003, Windows Vista,
Windows 2008 or Windows 7. The systems
must be networked.

See this blog post which explains some differences between the two programs.

There is a hardware solution as well, so called KVM (keyboard, video, mouse) switches:

A KVM switch is a hardware device that allows a user to control multiple computers from a single keyboard, video monitor and mouse. Although multiple computers are connected to the KVM, typically a smaller number of computers can be controlled at any given time."

Poor man's solution: Use a USB hub and plug your keyboard and mouse into that. Then run a USB cable from each computer up to the hub and plug in the cable for the computer you want to control.

Synergy2. It's a software KVM which works on just about any operating system. if both your computers are running Windows, I've found that Input Director is easier to get working.

"At work, I have two computers each with its own monitor (Windows 7, Windows 2003 Server).
The first one is controlled with the keyboard and mouse, the second one is controlled from the first one using Terminal Services.

This solution works pretty well for me and my coworkers (I'm a kind of evangelist of this poor man, but effective, solution)."

Maxivista will do this, plus it will also let you use the screen of the second computer in a multi-monitor configuration with your primary computer.