Adafruit seesaw is a near-universal converter framework which allows you to add and extend hardware support to any I2C-capable microcontroller or microcomputer. Instead of getting separate I2C GPIO expanders, ADCs, PWM drivers, etc, seesaw can be configured to give a wide range of capabilities. The ATSAMD09 breakout is great for development of seesaw capabilities (we use it in-house for our design work) or you can use it as-is to give your Raspberry Pi or ESP8266 more hardware support! Each breakout comes with the assembled and tested board, as well as some header strips.
Key Features

3 x 12-bit ADC inputs
3 x 8-bit PWM outputs
7 x GPIO with selectable pullup or pulldown
1 x NeoPixel output (up to 63 pixels)
1 x EEPROM with 64 byte of NVM memory (handy for storing small access tokens or MAC addresses)
1 x Interrupt output that can be triggered by any of the accessories
2 x I2C address selection pins
1 x Activity LED

Resources
Guide