There is a niche that is hardly catered for in the low-cost microcontroller world - the standalone device that can communicate with, say, an Android parent. Now one is about to hit the market - the Bluetooth enabled RFduino.
To understand why the RFduino is so exciting you have to consider how you might solve some very simple problems. Suppose you want to create a simple temperature monitor that reports its data so that it can be integrated into an environmental control system. You might want a few such monitors and you might want to add some additional control features in the future.
You might think at once that the solution is to use a standard Arduino - after all it is cheap and simple - but it only has a USB connector. If you want networking then you need to add an Ethernet shield, which pushes the price up and adds a wired connection to the system. You could go for a WiFi shield, but these are more expensive and power hungry.
OK, so switch to a Raspberry Pi which has built-in networking but no built-in WiFi. In this case you at least need a WiFi dongle and then you have to power it all. And don't even think about ZigBee because, while such radio connection are great for peer-to-peer devices, you don't want to add a weird radio board to a parent Android or PC.
RFduino is a tiny Arduino compatible board with a built-in Bluetooth 4.0 radio. It doesn't have a USB connector, but there is a USB shield that you plug it into to program it. Once programmed you can remove the USB sheild and communicate with the device over Bluetooth. What this means is that you have an Arduino compatible device that can, without the help of addons, communicate with any Bluetooth enabled parent - tablet, smart phone, laptop or PC. It is compatible with the Arduino Duo and Uno and you can use the standard IDE and download existing programs.
The RDduino has a 32-bit ARM Cortex-M0 processor, 7 GPIO lines which can all be used as either digital, analog, SPI, I2C, UART an PWM lines. It's power consumption is so low that it can run off a CR2032 button cell and its cheap at around $15 with a USB shield adding about $20. The plan is to create a range of stackable shields, much like the full size Arduino has available.
The whole project is on Kickstarter and, having reached its $20K goal within 4 days, is currently standing at just short of $250,000. The enthusiasm for the project is a measure of how much a device of this sort is needed. If you take a look at the Kickstarter page you can see the range of suggested uses, but most of them come down to wireless gadgets that with other hardware turn out to be too expensive.
Bluetooth was invented to get rid of connecting wires and RFduino puts this into effect for all manner of sensors and controllers.