I have good programming skills, but I know almost nothing about micro controllers. What would be a good place to learn more about these things, and what you can build with them? Would prefer something which teaches more about the electronics/hardware part than the programming one.
The Arduino is a different beast, you can't run Linux on it, it is a low-powered (by both meanings of the phrase) microcontroller meant to write real-time code which does most of the same things you'd use GPIO for on those other devices, it just happens to be better at controlling some things (like PWM-controlled devices) because you get more predictable timing because it isn't a full computer running a full multitasking OS).
Each of these devices have their own little communities, google "(product) forum" like "raspberry pi forum" to find good starting points for each device. The Adafruit learning system at http://learn.adafruit.com/ is a good place to get tutorials for lots of these kinds of devices (Arduino is heavily represented because it is the oldest, cheapest, and probably still most widely used of these platforms for creating custom hobby projects, but they have tutorials for just about everything).
http://hackaday.com/2013/04/15/florianh-shows-off-minimabl-t...
http://hackaday.com/2013/04/24/acrobatic-tricopter-inspired-...
http://hackaday.com/2013/04/15/glowing-easter-eggs-more-fun-...
http://hackaday.com/2013/04/20/mood-lampnotifier-uses-neat-m...
http://hackaday.com/2013/04/19/asynchronous-fireflies-use-fe...
http://hackaday.com/2013/04/17/upgrade-a-toy-keyboards-tone-...
http://hackaday.com/2013/04/23/otm-02-is-a-3d-printed-wristw...
http://hackaday.com/2013/04/24/this-gps-logger-is-so-small/
Some of them are just simple LED blinkers, others are more advanced such as UAVs. What are you interested in?
edit: just another link, great collection of projects done with Teensy, a development board bit like Arduino: