Use 2 computers with 1 mouse and 1 keyboard

QuickSynergy is a graphical interface (GUI) for easily configuring Synergy2, an application that allows the user to share his mouse and keyboard between two or more computers.
quicksynergy - Synergy in a single click - Google Project Hosting
Old hardware isn't expensive but what to do with all those computers? If you've ever tried using 2 keyboards side by side you know this isn't very useful. Having a server is nice but there isn't much point in it having a monitor.
Synergy is Free and Open Source Software that lets you easily share your mouse and keyboard between multiple computers, where each computer has its own display. No special hardware is required, all you need is a local area network. Synergy is supported on Windows, Mac OS X and Linux. Redirecting the mouse and keyboard is as simple as moving the mouse off the edge of your screen. Synergy is released under the GNU Public License (GPL).
Welcome - Synergy
Synergy is a program that lets you use 1 keyboard and mouse to control 2 or more computers
gimpy530 > e Synergy‬‏ - YouTube
Another solution is Maxivista. (this costs money)
MaxiVista turns any spare Desktop, Laptop or Netbook PC into a dual monitor for your primary computer. No extra multi monitor hardware is required. Simply extend program windows across multiple screens as if it were one big monitor. Increase your productivity by using multiple monitors.
Multi Monitor - Dual Monitor - KVM switch
Maxivista goes a bit further than just sharing mouse and keyboard, it supports drag and drop and one can actually drag applications from one desktop to another.
Use your Laptop as a second monitor. Control multiple computers from a single mouse and keyboard. Mirror your computer desktop to another screen
maxivista > - Dual Monitor Software‬‏ - YouTube
Combined with things like Wake on LAN and/or TightVNC/ultraVNC/realVNC (aka remote desktop) one can do truly miraculous things. At least in theory. :-)

Those trained in the arts know we should really be moving to dedicated processors. The industry hasn't taken off so the little people have no idea how wonderful this will be. Say, you want to run an irc client, email client, bittorrent client, news groups client and say an rss reader all at the same time but you also want to play games on your gaming rig. That 5 euro pentium 2 will do email just wonderfully. Some old mono-core pc for irc, your torrents and your news groups. etc etc Opening links is a bit awkward but I assume plug-ins can be devised for this.