Games with multi-monitor support
There are only very few games with explicit multi-monitor support, some examples are Microsoft flight simulators and Star Trek: Armada II. Some games were released with dualhead support exclusively for Matrox cards, see Matrox enhanced games.
If a game has a windowed mode, it is usually possible to play it on multiple monitors either by placing additional views on secondary monitors, or stretching the game window across multiple monitors. You might have to use software rendering for this to work. For more information, see the gaming-related links.
Microsoft flight sims support putting additional views on secondary monitors. To put a view on a secondary monitor, choose 'Undock window' and move it to the desired position. Starting with Flight Simulator 2004, views on up to 4 monitors are hardware accelerated, previous versions only used hardware accelerated 3D on the primary monitor. See Multi-Monitor Support in Flight Simulator 2004 for more on this.
With dualhead video cards or the triplehead Matrox Parhelia (surround gaming), it is also possible to play some games stretched across all monitors by using a stretched (spanned) display mode. This way, the game sees a single wide monitor instead of 2 or 3 monitors (the game needs to support widescreen display modes).