Summary + meta-summary from watching the Arduino community for a few years:<p>Because it's cheap, and because it's awesomely simple to use. It's a hobbyist's dream come true.<p>If you're interested in some of its uses, just look back through Make magazine's site. Loads and loads and loads of projects appear there.
And because it's so popular, there are a bunch of libraries and tutorials for it. It also abstracts or designs away a lot of the tedium in programing micro-controllers.<p>The net effect? It's like python for hardware.
At first, I loved it because it's pretty cheap, and I could just plug it into my USB port and start coding, without having to worry about making a programmer or buying a USB/Serial interface.<p>Now, I love it because it's pretty cheap, and it has full GCC toolchain support, so I don't have to use some proprietary IDE.
Cool, I'd been meaning to get an Arduino microcontroller myself.<p>Can anyone give examples of the more interesting projects that are being done with the Arduino that you know of?
The BeagleBoard is stuck in a no-man's-land between low-end 8-bit controllers like Arduino and the many other AVR/PIC platforms, and a full-fledged embedded PC. If I want to monkey with Linux and take advantage of a full 1 GHz ARM core, I probably want to just use a commodity PC motherboard. Chances are, it won't cost much more, and it comes with the huge advantage that I already know how to develop for it.