​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 ​here)
• A wide range of single board computers (see ​here)
• A standard stackable ​interface that lets you plug "shields" into the single-board computers to extend their functionality with things like motor control, Ethernet, WiFi?, etc.

You can even develop in the cloud (see ​here).

The development environment includes a large set of ​example programs (called "sketches" in Arduino parlance) and there are many excellent tutorials online including those from ​Adafruit, ​Sparkfun (in fact Sparkfun has ​many excellent tutorials), and even a fun video intro ​MIT.