One of the things I love about the Arduino platform is the huge number of different form factors that are available. This has got to be the smallest I've seen so far. It is TinyDuino.
The picture above shows some of the boards and a Starburst to give an idea of scale. The devices are, clockwise from the Starburst, processor, Bluetooth BLE, Accelerometer and programmer. The boards stack on top of each other using the tiny connectors.
When you want to program your device you put the programmer board into the stack and plug it into a USB port in your PC. It appears as an Arduino Pro-Mini clocked at 8Mhz (use the 3.3 volt option). It has all the pins of a standard Arduino uno and you can stack a tiny prototyping board onto the end of the stack if you want to directly access the signals. Once you've programmed your processor you can remove the programmer board to make your device even tinier.
There's a lovely little 16 LED array you can put on the end of the stack, and also a circular row of leds you can program up for a super-tiny cylon effect. There are also wifi, Bluetooth and even motor control boards you can add. The BLE device is a bit longer than the other ones, but you can still create really tiny devices with this platform. One version of the processor board has a clip for a lithium battery so you can easily create self-powered devices.
They work a treat and I strongly recommend them. However I would mention one word of warning. The manufacturer has recently changed their website and so all their documentation for the devices has moved around and is now very hard to find. If you need any help with the platform you are mostly on your own. Having said that though, most of the boards are based on very standard technology and so once you install the appropriate Arduino libraries they will work just fine.
I've been trying to get the Bluetooth BLE device to work with Windows 8.1 and not having a lot of success so far. I think this is down to my not understanding the way Bluetooth BLE works. The idea is to make a "ring of power" that I can wear so that I can send commands to my devices just by waving my hands. We shall see....