Android/iOS: If you want to control a Raspberry Pi or Arduino connected device from your phone, you'd usually need to know at least some programming. Blynk is an app that makes it easier to control those devices from your phone.
Blynk uses drag-and-drop widgets to help you control various parts of your electronics projects. This could mean LED values, servo speeds, or just checking in on a variety of data. You can check out the service for free, but you'll need to shell out some cash if you want to make your apps public or use it on a bunch of different devices. Most of us should be fine with the free plan though, at least to get started.
If you've ever wanted to connect your various electronics projects to your phone but didn't feel like going through the trouble of actually making an app, Blynk might do the trick.