Pro Micro 3.3V (Arduino compatible) ATMega32U4

This is a very small, and very capable board, which has a MicroUSB port on board to use for programming or for Serial input/output. It's well documented online, and is a very capable board, with plenty of digital and analog input/outputs. It can even be configured to emulate a Keyboard or mouse device.

Technical details:-

Specifications 
Size34mm x 18mm x 7mm (with header pins installed)
Processor8 Mhz ATmega32U4
Input Voltage5-9V Recommended.
Can be powered via USB
Digital Input/Outputs14 digital I/O pins (2 used for TX/RX)
6 PWM capable (D3, D5, D6, D9, D10, D11)
Analog Inputs4 x 10-bit resolution input.
Flash Memory32Kb (4Kb used by bootloader)

This board is not supported by the stock Arduino IDE, but can be easily added: Ebay does not allow us to provide a link to the instructions, but they can easily be found by searching for "pro micro Arduino IDE"

Once this is installed, you should select Board : "Sparkfun Pro Micro", and Processor : ATmega32U4 (3.3V, 8Mhz). If you accidentally program this as a 5V/16Mhz board, you will lose access to the USB port, this is recoverable, but can be tricky.

 

Thanks for your interest!