Around the CNET office, more and more people are using a two-monitor setup. It used to be a kind of luxury for data professionals -- people with lots of spreadsheets and documents that needed to be open side-by-side. But these days, with more people ditching desktops for laptops, hooking up an extra monitor is just a way to reclaim the screen real estate you lost by going mobile.
Now, maybe you considered a two-monitor setup years ago and got turned off by the idea of installing video cards and adapters. Those days are gone. With any reasonably new computer, you should be able to make this work with just a monitor cable, and a few minutes.
On any modern MacBook, iMac, or Mac Mini, there should be a little port that looks like the one shown at the center of this photo. Maybe it has a Thunderbolt logo next to it, or maybe it doesn't. Either way, it can be used as a generic Mini DisplayPort connection.
You can purchase a cable or adapter that goes from the Mac's Mini DisplayPort/Thunderbolt port to DVI or HDMI, and connect the DVI end to your monitor. Apple sells an adapter for $20, but I picked up this cable for less than $10 online.
Now, if your Mac's display port does have a Thunderbolt icon on it and you have some money to throw at a beautiful integrated solution, you can grab an Apple Thunderbolt Display and just plug it right in. The upshot to this solution is that the Thunderbolt connection can carry more than just audio and video. You can wire the display to Ethernet, FireWire, USB, and even other connected Thunderbolt displays, and all of that will carry over to your computer with just one included cable.
On your primary screen you'll see an arrangement tab for adjusting how the desktop will extend between the two screens. There's also a check box for mirroring the two displays, which can be useful for presentations. Also notice that you can drag the white menu bar in the Mac's Arrangement view to move your dock and menus to whichever screen you prefer.