What is this witchcraft? DeskDock, now available on the Play Store, allows you to share your computer's keyboard and mouse with your Android device. If you've ever used Synergy, it's very close to that.
What's the point of something like this, you may ask? The primary use the developer provided was to make Android development much easier. With this tool, you could work on an application on your computer, push it to your device, and test it without your hands ever leaving your keyboard. But there are plenty of other potential uses as well - you could use your Android tablet as another monitor to watch media on, for example.
So how does it work? Setting up DeskDock involves a little bit of effort, but once everything is installed, you shouldn't run into issues. First, you need to install the latest version of Java on your computer, then ensure you have the proper ADB drivers installed (on Windows, I use this), and finally make sure USB Debugging is enabled on your Android device.
Once all that is taken care of, open the DeskDock server app on your computer and the app on your phone. Plug your phone in via USB, allow USB debugging on your phone, and it should work automatically.
So how does it work in practice? Well, I tried it on two different Windows 10 PCs and two different Android devices. In all cases, the app worked just as advertised - my mouse could move between my PC and my phone with ease. Even when my phone's screen turned off, I could wake it by simply dragging my mouse to the phone.
DeskDock has a few more features that really make your Android device feel like an extension of your computer. Your clipboard is shared between devices, you can simulate multi-touch gestures with your mouse, and you can even connect multiple devices. DeskDock allows you to arrange your devices' screens at any position around your existing monitors, as seen below, just like native monitors on your computer.
The Pro version offers even more goodies, like typing on your Android device with your computer's keyboard, drag and drop for URLs and APKs, and additional shortcuts for adjusting volume/screen brightness.
DeskDock works on all Android 4.1+ devices, without root, and the server application runs on macOS (formerly Mac OS X), Windows, and Linux. You would be hard-pressed to find a pair of devices that this can't work with.