A very robust, simple to use micro controller.
From the Arduino Site:
Overview
The Arduino Duemilanove (”2009″) is a microcontroller board based on the ATmega168 (datasheet) or ATmega328 (datasheet). It has 14 digital input/output pins (of which 6 can be used as PWM outputs), 6 analog inputs, a 16 MHz crystal oscillator, a USB connection, a power jack, an ICSP header, and a reset button. It contains everything needed to support the microcontroller; simply connect it to a computer with a USB cable or power it with a AC-to-DC adapter or battery to get started.
“Duemilanove” means 2009 in Italian and is named after the year of its release. The Duemilanove is the latest in a series of USB Arduino boards; for a comparison with previous versions, see the index of Arduino boards
Summary
Microcontroller: ATmega168
Operating Voltage: 5V
Input Voltage (recommended): 7-12V
Input Voltage (limits): 6-20V
Digital I/O Pins: 14 (of which 6 provide PWM output)
Analog Input Pins: 6
DC Current per I/O Pin: 40 mA
DC Current for 3.3V Pin: 50 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
Be sure to check out the Arduino Page:
http://arduino.cc/en/Main/ArduinoBoardDuemilanove