The Arduino Nano is a small, complete  breadboard-friendly board based on the ATmega328. It has more or less the same functionality of the Arduino Duemilanove, but in a different package. It is powered with a Mini-B USB cable 
 
Specifications:
Microcontroller Atmel ATmega168 or ATmega328
Operating Voltage (logic level) 5 V
Input Voltage (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"
Length 45 mm
Width 18 mm
Weigth 5 g
 
Data Sheet:

Arduino Nano V3.0 clone

$20.00

Quantity

Microcontroller	Atmel ATmega168 or ATmega328
Operating Voltage (logic level)	5 V
Input Voltage (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"
Length	45 mm
Width	18 mm
Weigth	5 g