Unified Remote lets you control your computer in all sorts of neat ways, with only a slightly noticeable lag. The latter is very easy to forgive once you see the sheer number of specialized remotes the app includes. If you pay for the upgrade, the number of remotes it adds becomes almost absurd.
In order to make this program work, you need to download a companion program on the PC you want to control. There is a portable version of the program, so you can take it with you and put it on any computer you want. Windows Firewall may block some of the program's features at first, though. Once you've finally set everything up, turning on the Unified Remote app is a breeze. It can control your mouse, keyboard, and more, but it does take about a microsecond for your movement to register on your computer. The app includes custom remotes for programs like Spotify, VLC, and Windows Media Player. If you buy the full version, it gives you dozens more that have custom controls for just about any program you can think of. The premium version adds nifty voice controls, too. Sadly, even the full version of the app doesn't support third-party keyboards on your phone if you're a Swype or Swiftkey fan. Other than that, the app is very powerful and very fun to use.
If you're constantly running into situations where you need to control your computer from afar, download Unified Remote. The first couple of times you'll try this app, it will really feel like magic.