Three embodiments
The SAM 15x15 has three embodiments combined into one PCB:
28 x 28mm This board fits on a standard breadboard; it has 40 pins with 2.54mm pitch.
22 x 22mm This board is a smaller version of the 28 x 28mm board.
15 x 15mm This board has 40 pins with 1.27mm pitch.

Despite the small size, it has more I/O pins than the Arduino Zero or SAMD21 Mini/Dev Breakout: 34 instead of 21. Due to the very small dimensions it will fit easily into all embedded electronic projects, even in microdrones.
See more here:
http://www.avdweb.nl/arduino/samd21/sam-15x15.html

SAMD21G18 with pins
For hobbyists, the drawback of the SAMD21 is that it is only available in SMD. When blown up, it is pretty tough to replace it because no IC socket can be used. This can be solved with the SAM 15x15 board, which transforms the SAMD21G into a pin version.