Synergy+ is a cross-platform, open-source application that enables you to share a single mouse and keyboard with multiple computers -- somewhat similar to a KVM but without special hardware. Once installed on all systems, you'll have to pick which keyboard and mouse you want to share (server) and configure the secondary screens (clients) -- detailed instructions are available here. The initial setup can be a little confusing, but after you have everything up and running you'll be able to simply move your mouse to the edge of one screen and watch it seamlessly jump to another.
Of course, it's not the same as having a multiple monitor setup as you can't move programs or windows from one PC to the other, but you'll be able to multitask using two independent systems at once with the same mouse and keyboard combo. Another cool feature that Synergy+ offers is the ability to copy and paste data between your computers.
The program is based on and implements many bug fixes to the original Synergy project by Chris Schoeneman, which hasn't been updated in a few years. It's free and available for Windows, Mac OS X and Linux systems.