The PiGlow is a small add on board for the Raspberry Pi that provides 18 individually controllable LEDs. You can use it for all sorts of things! And of course, it fits inside a Pibow!
There are tons of things you can do with it:
Mood lighting / ambiance
Showing current system load
Notify you of events like mentions in tweets or incoming e-mail
Feedback the status of scripts/daemons running on your Pi
Works great when VESA mounted to provide a cast against a wall
...and pretty much anything else you can think of!
This board uses the SN3218 8-bit 18-channel PWM chip to drive 18 surface mount LEDs. Communication is done via I2C over the GPIO header with a bus address of 0x54 (Python example code provided). Each LED can be set to a PWM value of between 0 and 255. Comes fully assembled and ready to rock.
18 LEDs (three each of red, orange, yellow, green, blue, and white)
Great to provide feedback if you're running your Pi headless
PWM (dimming) control for each channel
Fully assembled (no soldering required)
Python code to access and control lighting provided