Search. It's been asked a lot.
Short answer: it's possible by buying something like the Matrox DualHead2Go. But that's expensive and not ideal, since your Mac will see both screens as one screen (menu bar will span both screens, dialog boxes and windows will pop up straddling both screens, etc). It's often more cost effective, if you consider how much it will be to get the Matrox thing and another monitor, to sell your Mini and buy an iMac instead. Then you can connect your existing monitor to the iMac, and it will be a true dual-display rather than a ghetto setup where two monitors are really only one display.