Arduino has a pretty good footing as the household name for beginners to get into electronics. Especially when it comes to that critical first task of blinking an LED, no other manufacturer makes it easier. Thanks to the power of today's microcontrollers, there is enough functionality in an Arduino to take a beginner and turn him or her into a pretty dangerous designer.
But what happens when the newly-accomplished engineer wants to expand beyond the hardware provided by the Arduino? Most people buy a shield which is a board designed to interface to the I/O pins of the Arduino, and some even design their own shields. However thanks to the Open Hardware nature of the design, there is a lesser-used method for customization: creating a 'derivative' AKA 'clone'.
A derivative/clone is a copy of the board with some design change(s) that still takes advantage of Arduinos development environment. The Arduino EAGLE design files can be found on the product page on the Arduino site and it is 100% allowed by the Open Hardware License to download them for any use, including commercial products. Many people have already designed and released their own version of Arduino by making a smaller or more capable board. These new products add extra functionality while still enjoying the benefits of Arduino's widespread adoption, ease of use, and vibrant community. And since the stock designs weren't optimized for small size, most have plenty of room for additional parts, especially something simple like a switching boost converter, high-power FET, or a high power RGB LED circuit.
Derivatives enjoy some significant design benefits over a shield. First, a single board is less than half the size and more robust than stacking an additional PCB on top of the purchased Arduino via headers. Second, it will be a cheaper solution than buying a stock Arduino and the custom shield PCB/parts. Finally, there is a lot to learn from starting with an existing PCB design and tweaking it. Not to mention the pride one would get from creating their own stand-alone design, even if it is attained by standing on the shoulders of giants.
Of course there are some drawbacks relative to designing a shield, starting with the extra hassle of purchasing and soldering the standard Arduino components on the custom PCB. The layout may also be harder because of the limited space and having to move some of the stock components around. And if the designer wants to share the functionality with others it is much easier to pop a shield on a stock board than having to build up a custom unit.
With all of the benefits, many budding engineers would be well served by chasing a derivative. Who knows? It could scratch a widely-felt itch and end up turning into a real product!