Adafruit 12mm Coin Cell Breakout w/ On-Off Switch
Simple but effective - this breakout board has a CR1220 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.
We also have a version without a switch