Adafruit 20mm Coin Cell Breakout w/On-Off Switch (CR2032)
Simple but effective - this sewable breakout board has a CR2032 coin cell battery holder soldered on, an on/off switch 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. There's two ground pins, one 'switched' power pin, and one pin that is always on.
Comes with one fully assembled and tested breakout plus a small stick of 0.1" header so you can solder it on and plug into a breadboard.
Coin cell battery is not included.