Eyefinity from AMD allows three monitors to run off of one Radeon 5000 series graphics card. Now, AMD has doubled the the numbers of monitors with Eyefinity6. Al you need for Eyefinity6 is one of the yet to be released 5000 series cards (the series is obviously available but Eyefinity6 isnt supported on existing cards yet), and of course the money to buy six compatible monitors.
In the above picture, Eyefinity is running on six Samsung ultra-thin bezel MD230 monitors. The total monitor setup goes for about $3100 (or $1900 for three).