@Max.Kcreated this Arduino Pro Mini compatible development board for low power applications:

LP Mini

ideal for data logging over extended periods of time [..] can run for months or even years on a coin cell

• Microcontroller: Atmega328p with Arduino bootloader
• Real Time Clock: MCP79410
• Voltage Regulator: MCP1700
• FDN340P MOSFET

• 2.8 µA current consumption in sleep mode
• Compatible to the Arduino Pro Mini (except for pin 2 and 10)
• A real time clock keeps the time and is used to wake the Atmega from sleep mode
• Sleep current is reduced by using a more efficient voltage regulator
• A simple Mosfet is used to turn external devices on and off if the IO pins cannot provide enough current