Is one monitor not enough for your needs? You can actually plug your Mac in to more than one monitor to enhance your productivity. Here's how:
Macs without Thunderbolt ports let you connect an external display via a built-in monitor port.
In simple terms and to cut a long story short, you’re typically limited to at most two displays via whatever arrangement you choose – with a MacBook or iMac this will be the built-in display plus an external monitor, while more recent Mac mini models allow you to connect two displays via HDMI and Mini-DisplayPort.
Thunderbolt on modern Macs changes the rules and you might be able to go one better and connect up two external displays, making for a total of three displays on a MacBook or iMac, but this depends on which model you have. See this Apple support document for details.
If you’ve a Mac with USB-C, such as the MacBook, then the rules are again different although at the present time we’re still waiting to see what USB-C hardware will become available.
However, for Macs that theoretically only allow a single external display there’s a few clever hardware hacks you can use to add-in more.
USB and Firewire for two or more displays?
Apple famously championed the Firewire standard a few years ago but you simply can't attach a monitor via Firewire. Firewire can receive video signals from the likes of digital video cameras but isn’t designed to output them.
Theoretically you shouldn’t be able to attach a monitor to a USB port either but a few companies have treated this as a technical challenge. The Matrox DualHead2Go and TripleHead2Go products let you connect up to two or three external displays respectively. They do this by augmenting the standard DVI/HDMI video output of a Mac or PC with data provided via a USB 2.0/3.0 connection. To learn if your Mac is compatible, check out Matrox's Mac compatibility listing, where you’ll also learn the maximum possible output resolutions - it's unlikely you’ll be able to run all three displays at 1080p, for example.
Diamond MultiMedia's BVU range lets you run a separate external display via nothing more than a USB 2.0 port. By connecting one to a MacBook Pro, as one example, you could utilise up to three displays: one built-in, one via the existing DVI/HDMI/DisplayPort, and one more via USB.
There are also some no-brand devices on eBay and Amazon that offer the same functions as the Diamond product, and are cheaper to boot - although be sure to check for Mac compatibility. Just search for something like "USB to DVI HDMI".
All the above solutions are somewhat hacky. We haven't tested any but we're certain performance won't be as good as a monitor attached directly. 3D gaming is definitely out of the question and video playback in anything other than standard definition (SD) will probably be choppy. Still, to put your email or Twitter app on a separate screen, as one example, they should suffice.
iOS apps like Air Display or Duet let you turn your iPad and/or iPhone into an external display for your Mac, letting you add a third or even fourth screen in addition to the built-in display and external display(s) connected via DVI/HDMI/Mini-DisplayPort/Thunderbolt.
In addition, don’t forget that modern Macs can output movie playback via AirPlay so if you simply want to watch Kill Bill while working on your existing Mac’s display(s) then an Apple TV attached to a TV or HDMI-compatible monitor is all you need.