A lot of people aren't aware that if they have an extra monitor laying around they can set up their iMac as a dual monitor setup. You don't even have to be very technical to do this and the setup is relatively easy and cheap. Setting up your dual monitor iMac should only take about 5 to 10 minutes max, depending on your setup and the type of iMac you own. Below is a general overview of what you need and how to do it.
For a dual monitor setup with your iMac almost any monitor will work with the display port converter as long as the monitor has a working DVI or VGA port. It is important to not that while most monitors will work with the display port converter not all will support the resolution size of the second monitor that you plan to connect to your iMac. As a rule of thumb your iMac monitor should always be as bigger then your secondary monitor, that way you don't run into any resolution issues.
All iMacs come with some type of display port connector. It is very important to know what type of display connector your iMac uses. There are two types of display ports, a standard and a mini. The newer iMac models use the mini display connector. In order for you to physically connect your second monitor you need to buy a separate converter cable that has either a standard display connector or mini display connector at one end and a VGA or DVI at the other end. You will need this display converter so that you can connect your video cables to your second monitor. Depending on which one you get, prices vary from $19 to $29 dollars if you buy from the apple store or apple online. I've seen after market display port converters on Amazon.com for cheaper but stick with the display converters made by apple they tend to work better.
Once you get the display port cables connected your iMac should detect the second monitor. If it does not swap out the cables or check your monitor settings in the system preference pane. To get to the system preferences pane click on the command key + the space bar and type in "system preferences" then click enter. From the system preferences pane choose monitor and see if your iMac detected your secondary monitor. If it does not then you might want to swap out the cable for to rule out a physical connection issue. Then try again from there. As you can see setting up dual monitors on your iMac is a snap. I promise that it will increase your productivity and increase your work space instantaneously.