Simple but effective - this sewable breakout board has a CR2032 coin cell battery holder soldered on and 0.1" pitch breakout pins for easy connecting. Great for powering very low-current projects, or adding a battery backup circuit to a real-time-clock or similar.
Comes with one fully assembled and tested coin cell battery holder and a small stick of 0.1" header so you can solder it on and plug into a breadboard. Coin cell battery is not included.
Adafruit also have a version with an on/off switch