The Arduino Nano is a great solution for projects that don’t have a lot of available space. It’s breadboard friendly, and has integrated USB.
It has everything that Diecimila has (electrically) with more analog input pins and onboard +5V AREF jumper. Physically, it is missing power jack and power select jumper. Since the Nano is automatically sense and switch to the higher potential source of power, there is no need for the power select jumper.
More about the Arduino Nano board