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Let’s say you change smartphones quite often. Instead of throwing the old one away, use it as an extra screen next to your PC, laptop or tablet.
There are many ways to attach a phone to a screen. I reused clips for smartphone lenses; Patafix (adhesive putty) works too.

Imagine three spare smartphones attached around your screen, hands-free, each running an app you would otherwise open on your main computer. I use YouTube, Hangouts, Messenger, Facebook, Slack, Trello and Gmail. Above the screen - a Hangouts video conference, on the left - Facebook, on the right - Trello with your notes. They all stay open all the time without taking space on the main screen, which usually shows the web browser.
Even one smartphone gives you a lot, and it can be the one you use every day.
The first option is tricky. Without a cable, high-resolution streaming lags a lot, and the software I found was too buggy to use. I’ve heard Apple has good solutions for that, but I only have Android devices.
The second option doesn’t stream the screen, so it works well over any connection: Wi-Fi, Bluetooth (Across Center) or a USB cable (DeskDock).

The shared clipboard is the key part: it glues all the devices together. Found something interesting? Move the mouse to Trello on the right and save it, or share it on Facebook on the left. You can even do it discreetly during a video conference ;).