Ready-Made Add-On Boards for Your Raspberry Pi
Many ready-made interface boards are available for the Raspberry Pi. These are designed to make things easier for you to do by building part of the circuits for you. They range from simply providing easier access to the GPIO pins, to including small sub circuits, to giving the Pi more inputs and outputs, or performing special functions not available on the Pi, like controlling the brightness of many LEDs.
You can always incorporate these sub circuits into your projects when you need them, but these boards provide a shortcut to some projects by building them for you.
Note that they are not essential and can be an expensive way of doing a project, mainly because they often contain more capabilities than you need for any one project. If you want to break up your project after you finish, you get value from these boards, but if you want to keep your projects and plug them in to show others, you're better off building what you need.
Dedicating a board to a project is an expensive way of doing things. However, some offer convenience that might be attractive to some people. New boards are constantly being developed and produced.
The Gert board for the Pi
The Gert board is the granddaddy of expansion boards. It is the closest thing there is to an official Raspberry Pi interface because it's designed by Gert van Loo, one of the Pi’s design team. It is not a Pi Foundation product. It is a compressive collection of interfaces, including an Arduino-like processor.
The Arduino is a standalone controller very popular with artists and engineers alike, it is superficially like a Raspberry Pi but is fundamentally a very different beast. It's better than the Pi at doing things that require very quick responses and accurate timing, but it has no display and can only be programmed in C++.
The Pi and the Arduino can work quite well together and so Gert has included one of these processors on his board. The board is designed for education, to give a flavor of different types of interfacing techniques. Basically, it's a kit of parts that you have to solder together yourself. Its features are
Twelve I/O ports buffered through 74HC244, each with an LED
Three push buttons
MCP4802: Two channel 8-bit D/A converter
ULN2803A: Six open collector channels up to 50V ~80ma/channel
ATmega328P: Atmel®AVR® 8-bit microcontroller (Arduino)
L6203: 48V 4A motor controller
780xx 3V3 low drop-out voltage regulator
It also contains the printed circuit board and the headers, jumpers, straps, flat cable, and sockets to connect to the Pi.
It's very improbable that any one project would need all these features and it's probably too advanced for the average beginner. But as you begin to explore this subject in depth, you might want to look at it.
The best thing about the Gert is that the manuals and build instructions are downloadable, so you can see in advance what you are letting yourself in for. You can read all about it and find the manuals’ download links on the Raspberry Pi site.
Pi Face for the Pi
The Pi Face board is designed by a team at the School of Computing Science at the University of Manchester (U.K.) and is aimed at the education market. It is roughly the same price as the Gert board, but comes ready-assembled. It is much less ambitious in scope but contains a good mix of things you would actually need for many simple projects.
These include onboard LEDs and push button switches for simple interaction along with two relays (physical switches moved by an electromagnet) for switching large currents. There are eight protected inputs and eight buffered outputs and the whole thing has screw connection access to connect it to the outside world. You can download a comprehensive list of documents and examples from Google Documents.
Other boards for the Pi
There are many other boards from small start-up manufacturers as well as web-based projects for you to build. You can find a good starting point for information on many of these on the Linux Wiki site.