Arduino and other Arduino-compatible microcontrollers are perfect when you need a small, rock-solid controller for your project. Program them, pop them into your project, and they'll happily run forever.
Arduino has a whole host of boards with different features and of different sizes, from the diminutive Micro right up to the feature-packed Mega. Adafruit's Feather range offers a range of different chips and a growing family of FeatherWing add-on boards.
For beginners, Arduino's Starter Kit is a great options, with everything from the Arduino board itself to components, and instruction booklets and project ideas.