The Nano (arduino compatible) is a small, complete, and breadboard-friendly board based on the ATmega328 (Arduino Nano 3.0) . It has more or less the same functionality of the Arduino Duemilanove, but in a different package. It lacks only a DC power jack, and works with a Mini-B USB cable instead of a standard one. The header pins are unsoldered.

Specifications:

Microcontroller    Atmel ATmega328
Operating Voltage (logic level)   5 V
InputVoltage (recommended)     7-12 V
Input Voltage (limits)                   6-20 V
Digital I/O Pins                              14 (of which 6 provide PWM output)
Analog Input Pins                          8
DC Current per I/O Pin               40 mA
Flash Memory    16 KB (ATmega168) or 32 KB (ATmega328) of which 2 KB used by bootloader
SRAM                                           1 KB (ATmega168) or 2 KB (ATmega328)
EEPROM                                       512 bytes (ATmega168) or 1 KB (ATmega328)
Clock Speed                                16 MHz
Dimensions                                 0.73" x 1.70"

Power:
The  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.

The usb chip on the Nano is only powered if the board is being powered over USB. As a result, when running on external (non-USB) power, the 3.3V output (which is supplied by the usb chip) is not available and the RX and TX LEDs will flicker if digital pins 0 or 1 are high.
Memory