Radios and Microcontrollers
Cut the cord & connect everything!
Bluetooth 4.0 / LE
Bluetooth LE is excellent and many boards have been released around the TI CC2540.
BLE Shield and Mini
From Red Bear Lab, lots of example code and open source Biscuit firmware.
Jinan Huamao Technology makes inexpensive Bluetooth LE modules (HM-10, HM-11) based on the TI CC2540.
Datasheets, sample code, etc