Adalogger FeatherWing – RTC + SD Add-on For All Feather Boards

It works with UNO, Mega, Leonardo, NG, Diecimila, Duemilanove, and compatible Arduinos. Yun’s and Arduino Ethernets have a chunky Ethernet jack that gets in the way of stacking, you can use the stacking headers included and it will work, just doesn’t sit nice and flat.
Resources

It has a nice standard 0.1″x0.1″ prototying grid with big pads
Comes with Stacking headers and plain header, choose whichever you want when soldering together
A IC pattern for adding DIP ICs up to 20 pins
Power rails down the middle and sides
A reset button and an extra general use button
2 3mm general use LEDs, red and green, as well as 2 matching resistors
A pass-thru ICSP stacking header so you can stack any kind of shield on top, and/or use an AVR programmer
A surface-mount chip area for up to 14 SOIC size parts
Compatible with tiny breadboards
Every pin is brought out!
Gold plated pads
2 x 0.1uF capacitors on either side for extra power stability

Resources
Guide