Dual monitor iMac?

Using Mini DisplayPort output with support for DVI, VGA, and dual-link DVI (adapters sold separately).

So if your monitor is VGA or DVI, you need to get an adapter.

Cube Over is correct if you want to use a third party display, however if you want to use one of the new Apple Thunderbolt Display you can connect via Thunderbolt. The advantage of that is you can also daisy chain other Thunderbolt devices to some extent. If you are interested in the display click:

http://www.apple.com/displays/

If you would like to read about daisy chain capabilities please look over:

http://support.apple.com/kb/HT4885

or

http://www.tuaw.com/2011/09/18/thunderbolt-display-daisy-chaining-spec-cleared-u p/