ATMEGA328 (more flash and ram memory)
Blue power LED is on the top
A0-A7 pins compatible with Arduino Stamp and Pro Mini
Two layers PCB
Easier to hack the Eagle file
What is Arduino?
Arduino is an open-source electronics prototyping platform based on flexible, easy-to-use hardware and software. It's intended for artists, designers, hobbyists, and anyone interested in creating interactive objects or environments.
Arduino can sense the environment by receiving input from a variety of sensors and can affect its surroundings by controlling lights, motors, and other actuators. The microcontroller on the board is programmed using the Arduino programming language (based on Wiring) and the Arduino development environment (based on Processing). Arduino projects can be stand-alone or they can communicate with software on running on a computer (e.g. Flash, Processing, MaxMSP).
Arduino received an Honory Mention in the Digital Communities section of the 2006 Ars Electronica Prix. Credits
Arduino Nano overview:
Arduino Nano is a surface mount breadboard embedded version with integrated USB. It is a smallest, complete, and breadboard friendly. It has everything that Diecimila/Duemilanove has (electrically) with more analog input pins and onboard +5V AREF jumper. Physically, it is missing power jack. The Nano is automatically sense and switch to the higher potential source of power, there is no need for the power select jumper.
Nanos got the breadboard-ability of the Boarduino and the Mini+USB with smaller footprint than either, so users have more breadboard space. Its got a pin layout that works well with the Mini or the Basic Stamp (TX, RX, ATN, GND on one top, power and ground on the other). This new version 3.0 comes with ATMEGA328 which offer more programming and data memory space. It is two layers. That make it easier to hack and more affordable.
You end up paying less with Nano than Mini and USB combined!
Operating Voltage (logic level)  5 V
Input Voltage (recommended)  7-12 V
Input Voltage (limits)
Digital I/O Pins
14 (of which 6 provide PWM output)
Analog Input Pins
DC Current per I/O Pin
32 KB (of which 2KB used by bootloader)
0.70 x 1.70
Automatic reset during program download
Power OK blue LED
Green (TX), red (RX) and orange (L) LED
Auto sensing/switching power input
Small mini-B USB for programming and serial monitor
ICSP header for direct program download
Standard 0.1 spacing DIP (breadboard friendly)
Manual reset switch
The Arduino Nano can be powered via the mini-B USB connection, 6-20V unregulated external power supply (pin 30), or 5V regulated external power supply (pin 27). The power source is automatically selected to the highest voltage source.