This is a super low-cost way to drive 18 LEDs at constant current up to 34mA per channel- no resistors required! Just hook up the cathodes of your LEDs to each channel and provide a common 5V supply for the anodes and away you go!
The I2C interface is very simple to use and works with Raspberry Pi, Arduino, and many other platforms - the device address is 0x54. With the Raspberry Pi you can use the 3V3 supply to power the chip (via the VCC pin on the breakout board) and the 5V supply to power the LEDs.
You can pick your own LEDs and layout to suit your project, and then use the PiGlow library and existing examples to get you started.
We have an Arduino library available here: https://github.com/pimoroni/pimoroni_arduino_sn3218
And a Python one for the Raspberry Pi, too: https://github.com/pimoroni/sn3218