Arduino & Clones
The Arduino Uno is probably the most popular microcontroller board of all time. The first Arduino's were invented and became available in 2006 and were in kit form, mainly for use in technology type classes. The creators were intent on making them as user friendly as they could possibly be, eventually coming out with the Uno model with its distinctive color and shape which made it easily recognizable.
The company has an interesting history which I will not try to recount here... just check out their own Arduino Uno history page.

The Arduino programming language is similar to C or C++ but is simplified somewhat to make it easier to use. Also, there are countless 'libraries' available one can include in their programs (or "Sketches") which makes it really easy to quickly come up with a usable program in short order. Then, to top that off, there are so many forums and other help resources for Arduino that you can't help becoming expert in a short amount of time. Well, at least if you have some imagination and a measure of technical ability, but that doesn't take long either if you're really interested in this as a hobby.

In fact the Arduino hardware platform is SO popular that it has been cloned all over the world in so many forms it's hard to keep up with it. I have some Arduino compatible boards (they come in many forms) that are about the size of a US postage stamp! I've had need of that tiny size for a project or two so far and have several of them in my collection waiting to be used when needed. If you look around online you will find very many boards that let you program them using the Arduino IDE, which is quite the compliment to the original developers.

There are also a vast number of 'shield' boards that are designed to stack on the Arduino Uno style board using their distinct I/O connector arrangement. It becomes a vertical buss of sorts. These boards can offer something as simple as I/O breakout to phoenix connectors, to complicated interfaces for any application you can come up with. There are even proto boards you can solder your own designs onto and stack them on the Uno.

All this is why these have become soooo popular.