Two years ago I found the MEDUINO in Ebay (5 pieces for $44.95 including shipping).
Since then I have used them in several projects with good results.
For less than $10, you have an Arduino NANO compatible device with selectable working voltage (3.3V or 5V), additionally, you can use it as a USB-Serial adapter (FTDI chip) to interface or monitor other serial devices.
Meduino 3.3V capturing data from the ADXL335 accelerometer.
Meduino 5V transmiting light intensity from the Single Pixel Camera to Processing.
Meduino 3.3V as a USB-Serial adapter to check MODBUS protocol with the electric imp.
Meduino 3.3V as power source and USB serial adapter for the ESP8266 Wi-Fi module.
The pin arrangement is like the Arduino NANO but with the Mini-USB connector on the opposite side. The PCB of the Meduino is also slightly larger.
Meduino NANO and Arduino NANO pin assignment
The selectable Vcc (3.3/5V) is great to interface with different sensors and hardware.
One thing to consider for “serious” applications is the crystal frequency at 3.3V.
The ATMega328 datasheet shows a Safe Operating Area, the maximum recommended frequency for 3.3V operation is 13.3 MHz.
The Meduino NANO with 3.3V at 16MHz is out of the recommended frequency but for experimenting purposes at room temperature, is a very handy and low cost Arduino.