KiCad is an EDA software suite for the creation of professional schematics and printed circuit boards. Kicad runs on Windows, Linux and Apple OS X and is released under the open-source GNU GPL v2.

This is an open source AVR-based microcontroller board that excels due to its ability to precisely control things like motors, inputs, …, and its library that eases programming. Arduino comes in different versions, ranging from 8 to 32 bit, 16Mhz to 81Mhz, supporting as many as 54 digital IO pins of which 12 can be used as PWM. An Arduino runs only one program (from its internal flash), so this has complete control over it.

A power source is the only thing needed to run it, and it even output a stabilized 5V/3.3V that can e.g., be used directly to power a few LEDs.

There exist a lot of shields, which are basically prints that can be stacked on an Arduino to extend it with some specific piece of hardware.

A Raspberry PI is a cheap but full fledged Linux-able ARM-based computer. It has a few basic IO pins and even a PWM pin, and is has a serial port that can be used for interfacing with e.g., an Arduino.

This is a H-shaped formation of 4 transistors that interface a DC brushed motor with a PWM source.