I often have the need to use multiple monitor setups but don't always have access to a dedicated spare screen. Maxivista allows you to "steal" another monitor attached to a machine on the network to use as your second monitor.

You just run a stand alone executable file on the slave machine and use its screen as your second screen over TCP/IP. Very sweet! You can even remote control the second machine. I also use UltraMon with Maxivista to give me a enhanced features like window "flicking" and split taskbar.