Setting up a special secondary display on iMac
The operating system supports multi screens. For the iMac i would say that a DisplayLink adapter would allow you to connect additional screens via USB. I've never used it, so I have no idea what the maximum resolution is, but I read several times on different boards that this solution has been successfully used.  

Update: As gabesubdo mentioned in a comment below, it is not necessary to use a DisplayLink if your iMac has one or more Thunderbolt ports, which can be used as Mini Displayports as well. Apple provides adapters for Mini Displayports (to HDMI, VGA, ...)