I'm not sure where to post this, but here goes ...
Below is a modified photo of the Uno Rev3 board, with annotations about the extra meanings of some pins (not printed on the board):
I personally find that when trying to hook up I2C or SPI, I can never remember which pin is SDA and which is SCL. Hopefully this photo will help others to quickly connect up their systems.
Earlier boards are similar except that they don't have the ICSP header for programming the USB interface chip, and the extra SDA/SCL sockets for use with I2C.
I find that the pins on the ICSP header (on the right) are useful for grabbing an extra +5/Gnd combination if I need to power other devices (or for hooking up a multimeter or logic analyzer).