DIY web site Instructables has a very intense but very cool tutorial for converting a laptop into a second monitor.

This is a “roll up your sleeves and hope you don’t screw it up” project, but it’s also a great way to repurpose an unused laptop for a cheap second LCD monitor. For a non-DIY approach, you can use a software like MaxiVista to transform a second laptop without the hardware tweaking, but the Instructable will take the repurposing to the limit. On the other hand, if you’re just looking to share a mouse and keyboard, check out previously mentioned Synergy.