Feather is the new development board from Adafruit, and like its namesake it is thin, light, and lets you fly! Adafruit have designed Feather to be a new standard for portable microcontroller cores.
At the Feather 32u4's heart is at ATmega32u4 clocked at 8 MHz and with 3.3V logic, a chip setup Adafruit has tons of experience with as it's the same as the Flora. This chip has 32K of flash and 2K of RAM, with built in USB so not only does it have a USB-to-Serial program & debug capability built in with no need for an FTDI-like chip, it can also act like a mouse, keyboard, USB MIDI device, etc. This chip is well supported in the Arduino IDE and can run just about every sensor/library out there.
To make it easy to use for portable projects, Adafruit added a connector for a 3.7V Lithium polymer battery and built in battery charging. You don't need a battery, it will run just fine straight from the micro USB connector. But, if you do have a battery, you can take it on the go, then plug in the USB to recharge. The Feather will automatically switch over to USB power when its available. The battery is also tied thru a divider to an analog pin, so you can measure and monitor the battery voltage to detect when you need a recharge.
The Feather 32u4 Basic Proto has some extra space left over, giving you a tiny little prototyping area. If you just need to attach a button or sensor, you may be able to skip out on a breadboard and wire it directly on there.
Comes fully assembled and tested, with a USB bootloader that lets you quickly use it with the Arduino IDE. We also toss in some header so you can solder it in and plug into a solderless breadboard.
Lipoly battery and USB cable not included.