The one I'm about to describe has been developed by Adafruit as a kit and is called the Boarduino. It's cheaper than the standard Arduino, and has a smaller footprint (about a third of the size, less than an inch wide by about 3 inches long), so it could be useful where your project needs to be compact. It can also do something an Arduino can't - plug in to a breadboard.
You can't have enough 'uinos, so I built the Boarduino kit, again a satisfying experience:
Notice that this photo shows an ATMega 168 chip - they now actually come with an ATMega 328 chip, which has twice as much memory, and comes pre-loaded with special bootloader software. I intended to have this on a remote project, communicating remotely with my main Arduino Uno R3 - more on this later.
There is a 2.1 mm socket on the Boarduino to allow power to be connected, and there is (on the left of the board above) a 6-pin header for programming, which also takes in power from the USB supply. You wouldn't use both power inlets for obvious reasons, so when you have your Boarduino programmed with a sketch from the IDE, you can disconnect the USB end, and run it on power through the 2.1 mm plug. Its on-board 5 V regulator allows you to use anything from 7V to 17V through the 2.1 mm plug. Don't forget to move the jumper on the board which selects one power inlet or the other.
Because the Boarduino doesn't have a chip on board enabling USB communication, I also got an FTDI cable. This is more than just a USB to 6 wire female header connector - it has embedded within its insides, a special chip (it must be very small).
This cable connector allows the Boarduino to be connected to your PC USB port, essential for transmitting programs (sketches) to it, from the Arduino IDE software.
I found this cable to be very useful later for programming other 'uinos - as will be seen in a later post.