Atmel's ATMega328P 8-Bit Processor in 28 pin DIP package. It comes with 32K of program space. 23 I/O lines, 6 of which are channels for the 10-bit ADC. Runs up to 20MHz with external crystal. Package can be programmed in circuit. 1.8V to 5V operating voltage! The famous Arduino Deumilanove, and Arduino UNO is using this ATMega microcontroller.
Anyhow, this microcontroller is fresh new without any bootloader.
Good news! Start from August 2012, this microcontroller is pre-loaded with Optiboot bootloader (Uno 16MHz). This means you can use Arduino code in your own project without using an actual Arduino Board. No programmer is needed anymore but you still need UC00A and basic circuit. Please select Arduino Uno Board in the Arduino IDE. We are using this microcontroller in our Breadboard Friendly Arduino (BBFuino). Therefore, you can refer to BBFuino's schematic and tutorial for more information.