Dedicated Arduino Nano V3 I/O expansion board.
Can be mounted on Arduino Uno too, like a shield after extra headers are soldered to pads along edges of the board.
All Arduino pins both analog and digital conveniently exposed with +5V and GND in the format commonly used in servo motors and sensor breakout boards.
Besides that:
- the separate header for serial port again with +5V and GND to conveniently connect TTL to RS232 or USB converter. (D0 and D1 pins)
- the separate header for i2c  bus (A4, A5,+5v, GND)
- header with +3V,GND pair from Nano onboard regulator.

When you use a power extension board, the maximum current output can reach 5V/2A.

Red board uses coloured pin headers: Red 5V, black GND, yellow I/O, white COMS.
Beginners might find this helpful while some might find this noisy and annoying. For those, we have blue board.