Dual monitor setup in Linux has never been easier. While methods such as the xinerama extension sometimes drive people insane, using RandR (Resize and Rotate) is quick and painless. This will allow you to use both monitors as one big screen instead of two identical ones (cloning). Follow these simple steps to get started.
Note: This HOWTO assumes that you are using the opensource drivers for your video card (ATI or Intel). Nvidia and AMD proprietary drivers come with their dual screen components.