SoftTH
SoftTH is a specialized modification for DirectX that allows you to span your games across 3 monitors. With strategy games, it allows you to see a much larger portion of the battlefield, while in flight sims, FPSs, and driving sims it allows you to see much more at once.
To use SoftTH, all you need is 2 graphics cards and 3 monitors. The first card needs to be relatively powerful, since it handles all the rendering. The second card just needs to be Direct3d Compatible, since all it does is pass the information to the monitors
The hack is compatible with most games that use DirectX, however it may take some playing around in order to get it to work correctly.
Before trying to set up games according to the following instructions, make sure that you have the cfg file that is included with the download set up correctly. It comes with a quickstart guide that does a pretty good job of walking you through the steps.
You can download it here
Game List
Here is a list of games that I've gotten it to work on.