As either an experiment in parsimonious project creation or a bare data logger - this project by Instructables user "RayBurne" fits the bill. They've used the absolute, below-specification application of an ATmega328P-PU with the Arduino bootloader as the heart of the simple device, which simply records incoming data from an RS232 stream and writes it to an SD card. You may damage some parts in the process so take care and heed all the instructions. All the instructions and sketches are included so with some time you can reproduce it yourself.
If you're wanting to reproduce the project above - we've got you covered with our new ATmega328P MCUs with Arduino Uno bootloader:
This is the same Atmel AVR ATmega328P microcontroller used in the official Arduino Uno, as well as our Eleven, EtherTen, USBDroid, and other boards. Perfect for building your own Arduino-compatible project directly on a breadboard or on a custom PCB, or for replacing the MCU in an existing board. Comes with the Arduino Uno bootloader pre-installed. Better still, it even has a special label stuck on top with details of the pinout, so you don't even need to look up the datasheet when connecting it up in your project! For more information and to order, click here!