The HC-05 Bluetooth module is the perfect way of adding Bluetooth functionality to your project. It easily allows you to establish communication between your Arduino and other devices, including other Arduinos, PCs, phones and tablets.
The HC-05 comes in pre-soldered with a 6-pin 0.1" female header, and can be plugged straight into a breadboard or 0.1" female header Dupont cables.
NOTE: although this module can be powered with 5V (the HC-05 has an onboard voltage regulator), the key and communication pins operate on 3.3V. To achieve this with devices that output 5V (such as an Arduino Uno), you can either use a logic level converter or a voltage divider circuit (uses 2x resistors per pin) to bring down the voltage. (You can use resistors in series with the pins, but determining the correct value is trial-and-error).