I work from a home office where I've got a slick 3-monitor setup... three 27" monitors, two sitting horizontal at 2560x1440 and one sitting vertical at 1080x1920 in the middle (by the way, a center vertical monitor is awesome for coding). All this is run off the two Thunderbolt & single HDMI ports on my MacBook Pro (late 2013):
I've become so dependent on this setup that when I travel or work outside of my office, I avoid some tasks when I don't have a second monitor.
However, lately I've adopted an awesome 2nd monitor solution for my laptop that doesn't require much extra stuff. When I work outside of my office, I always have my iPad (Air 2) with me. Recently some ex-Apple engineers launched a duet for only $15.99, an app you buy for your iPad that comes with a free companion Mac or PC app. When you fire duet up on your laptop & iPad, your iPad is seen as a second device. It communicates over the super fast USB3 cable from your laptop to your iPad... and it works great! At first I noticed some lag, but recent software updates fixed that.
But you had your iPad sitting on the table which was always a little lower than your laptop screen. Well, enter Mountie. This little $20 device quickly connects your laptop and iPad together & even angles the tablet in a bit for easy viewing. I love this setup... huge thanks to my buddy Ben Stegink for pointing this out! You can orient it either in portrait or landscape mode... and best of all, it only added a tiny little device to my backpack... it will even fit on a plane! Now that's awesome!