But with the Raspberry Pi, one of the coolest things is that you can use the GPIO pins to interface with other bits of electronics, and my second Pi (the first is my Media Centre at home running RaspBMC) is now set up with a breakout board and case to make it a little more robust.
|Not sure why this uploaded upside down....|
Here is a closeup:
You will notice that there are 8 GPIO pins broken out (P0 to P7) as well as the UART, I2C, SPI, etc. I found it difficult to find detailed information on which pin on the breakout corresponds to which BCM GPIO pin. I traced them, and being the nice sort of fellow I am, I'll detail them for you below. Bear in mind this is a Rev 2 Pi and this board is labelled "Raspberry Pi GPIO Extension Board V2.2", so if you don't have those precise items, YMMV.
|Breakout Pin||BCM Pin||Notes|
|P1||GPIO18||Also PWM pin.|
|P2||GPIO27||Was GPIO21 on Rev1 Pi.|
|P7||GPIO4||Also the 1Wire pin by default.|
|I2C SDA||GPIO2||Was GPIO0 on Rev1 Pi.|
|I2C SCL||GPIO3||Was GPIO1 on Rev1 Pi.|
I hope the above is useful to you, and helps you get the most out of your Raspberry Pi GPIO experimenting!