This is made possible through a smartphone app and PC app combo called Unified Remote. The app on your phone connects to the server app on your PC, which allows it to send mouse, keyboard, and other remote control-type input.

YouTube’s Pairing feature offers a way to use your phone as a remote to control YouTube playback on your computer, and there’s even a way to use your phone as a remote control for playback in VLC. But these are very limited remote control solutions. The ideal would be a way to use your smartphone as a wireless mouse and keyboard.

Unified Remote works very well for this. The standard version is free, with a paid version offering additional specialized remote functions.

The free version will let you use your phone as a mouse, keyboard, and give you access to other media remote functions. You can install the app on an iPhone, Android phone, or even a Windows Phone. You can use it to control a Windows, Mac, or Linux PC. So whatever devices you have, Unified Remote should work for you. It can communicate with your computer using Wi-Fi or Bluetooth.
Install the Server on Your Computer

First, you’ll need to install the Unified Remote server software for Windows, Mac, or Linux. You’ll find this software on the Unified Remote website. Download it and go through the installation process. On Windows, this installs an input driver that allows Unified Remote to control your computer

The installer should automatically launch Unified Remote after it’s finished. If it doesn’t, launch the “Unified Remote” application from your Start menu. It’ll appear in your notification area, and you can right-click it and select “Manager” if you want to adjust its settings–but that isn’t necessary

The process should be similar on Mac OS X and Linux. Just follow Unified Remote’s installation instructions.
Install the Remote on One or More Phones

Next, you’ll need to install the Unified Remote app for Android, iPhone, or Windows Phone. Launch the app on your phone and tap the “I have installed the server” button. The app will scan your local network to find a computer running the server, so be sure your phone is on the same Wi-Fi network as your computer.

Once it’s done, it’ll give you a list of remotes you can select

The “Basic Input” function works well, allowing you to use your phone’s screen as a trackpad to control the mouse cursor. This can be much more convenient than attempting to use your mouse on the coffee table when you want to move the cursor around on your media center PC.

Other common trackpad actions like a single tap to click and a two-finger drag to scroll up and down will also work. From the Basic Input screen, you can tap the keyboard icon at the bottom-left corner of the screen to pull up your smartphone keyboard. Type on the keyboard and it’ll send that input to your computer

Other remote control functions can also be useful. The File Manager remote allows you to manage files on your computer, while the Keyboard remote gives you a full keyboard–useful if you want to use keys that don’t appear on your smartphone keyboard, like the Windows Key.

The Media remote could be extremely useful allowing you to quickly send volume down, mute, volume up, previous, next, stop, and pause/play key presses to your computer. The Power remote allows you to quickly restart, shut down, sleep, lock, log off, or hibernate your computer

There are a few other free remotes you can add by pressing the Plus button. For example, there’s a Start remote that lets you quickly launch apps from your Start menu, a Task Manager that allows you to manage running processes, and a VLC remote that provides buttons for quickly controlling playback in the VLC media player.

Many of the other specialized remotes here cost money, and that’s how the app makes a profit and can afford to offer all the basics for free. You don’t need to spend any money unless you want the fancier stuff

It’s that simple. There are a wide variety of other apps you can use for this, but they’re not necessarily as full-featured and cross-platform. Unified Remote provides a server that functions on every common desktop operating system–Windows, Mac OS X, and Linux–and smartphone apps that work on iPhone, Android, and even Windows Phone.

More than one device can be connected to the server, too. Other people in your house just have to install the app on whatever smartphone they’re using and they’ll gain the ability to control your computer, as long as they’re on the same network.