[https://www.arduino.cc/ Arduino] is a wildly popular hobby computing platform.  Arduino is open-source and includes:
  * A friendly software development environment that runs on Windows, Mac OS X, and Linux (get it [https://www.arduino.cc/en/Main/Software here])
  * A wide range of single board computers (see [https://www.arduino.cc/en/Main/Products here]) based on AVR and ARM processors
  * A standard stackable [http://www.gammon.com.au/forum/?id=11473 interface] that lets you plug "shields" into the single-board computers to extend their functionality with things like [http://www.gearbest.com/other-accessories/pp_218079.html motor control], [http://www.gearbest.com/development-boards/pp_136559.html Ethernet], [http://store.cutedigi.com/juniper-wifi-shield-for-arduino-based-on-gainspan-module/ WiFi], etc.

You can even develop in the cloud (see [https://codebender.cc/ here]).

The development environment includes a large set of [https://www.arduino.cc/en/Tutorial/HomePage example programs] (called "sketches" in Arduino parlance) and there are many excellent tutorials online including those from [http://www.ladyada.net/learn/arduino/ Adafruit], [https://learn.sparkfun.com/tutorials/what-is-an-arduino Sparkfun] (in fact Sparkfun has [https://learn.sparkfun.com/tutorials/tags/arduino many excellent tutorials]), and even a fun video intro [http://video.mit.edu/watch/arduino-tutorial-1-10950/ MIT].