Of course, nothing's more affordable than gear you already have--like, say, an old laptop that's just collecting dust in a closet.
MaxiVista is a clever utility that lets you turn a second PC into a second monitor. As long as both systems are connected to your home network, you're good to go.
Most users will probably pair their desktop with a laptop, though you can just as easily configure two laptops, a laptop and a netbook, and so on. The software now supports third and fourth PCs as well.
I put MaxiVista to the test on a desktop system running Windows 7 and a laptop running Vista. It worked flawlessly. Even Windows 7 features like Aero Snap worked on the secondary system. Very impressive.
MaxiVista costs $40. There's a 14-day trial version available, which you should definitely install first to make sure your configuration works properly. I think it's a terrific solution for anyone looking to put an old or unused PC to genuinely good use.