Wi-Fi Video Tethering
Basically, you use a 3rd party application to remotely show an extended display. There are both iOS and Android solutions. You can use either your phone or tablet as an extended monitor.
Pros: Cheap $4-10.
Cons: Slow, Laggy, Cost does not include tablet. Wifi may be an issue.
You need to factor in the cost of your tablet and phone. Importantly, you need to be on the same Wi-Fi subnet. This works at home and at some work locations but forget about public cafes. I've notice some coffee shops issue DHCP address on different subnets and some only give wifi access for only one device. You wont be able to use this a your local Starbucks or Peet's coffee.
You can buy various USB monitors using displaylink technology. The above is a 7" portable MIMO monitor.
Pros: Moderate price. Screen size range from 7" to 21"
Does not require external video port. If your laptop or ultra-book doesnt have a video out (HDMI/Displayport), this can be a handy option. Also, you only have to deal with one cable. A USB cable provides both power and display.
This also comes in handy if you want to add extra monitors to an already max out display option. For example, my Thinkpad can only output to one single external display. The displaylink allows me to add extra displays (a 3rd and 4th) even though I only have one Displayport out. I've chained 4 DisplayLink monitors in the past and it is quite useable for static content like your email in a separate monitor.
Cons: Laggy. Requires USB drivers. Furthermore, it will not work with all operating systems. Consumes CPU resources. The biggest con is it requires a driver. You simply cannot plug it into a PC stick or old computer and expect it to work. It is not recommended as a primary display. For example, you should not think of using this if you have a Linux PC server tucked in the closet. If you wanted to plug in a monitor to diagnose that old PC you have lying around, forget about it. And if you are a Debian/Ubuntu Linux user, setting up DisplayLink is big hassle.
Portable MHL/HDMI monitor
I recently reviewed a 15" MHL/HDMI monitor, GeChic monitor that is completely battery powered. This is in essence a real portable monitor. You can even flip it in portrait mode as shown in the picture above.
There are a few in the market place ranging from 7" to 15". In fact, there are some that are basically tablet form factors and offer additional ports such as MHL that will provide display and power to your smartphone.
Pros: Work with any device using HDMI, DVI, MHL and VGA(with adapter)
Cons: Costly. Delicate and require attention to protect the screen when packing.
Hacked solutions like LapDock (original Atrix LapDock)
Original Link: http://fortysomethinggeek.blogspot.com/2013/05/cheap-portable-hdmi-monitor-for-your.html
I originally got myself a LapDock 100 which didn't work. The lapdock to get is the original Atrix 4G Lapdock.
Pros: Cheap. I've seen them in discount bins as low as $20. However, expect to pay $50-80.
You also get the addition of keyboard/mouse with Lapdock. They fold up like traditional netbooks so you get the added protection of the screen. You can in essence plug this to anything with a HDMI out. If you have a game console or DVD player, you can use this a monitor and sound is routed through HDMI.
Cons: Requires hacking. You'll need to splice USB cable to prevent back-feeding. The USB out sends out power which may overload your host. It is adviseable to cut the +5V red line to prevent harm to your devices. Requires multiple adapters.
There you have it. A few portable monitor solutions. I am personally in love with the Lapdock solution. The lapdock is incredibly thin and the keyboard base acts as a screen cover. Unlike the GeChic, I don't have to worry about throwing it inside my backpack. The battery life is great at 8 hours. And out of all the solutions, the LapDock provides the best picture color quality.