When you first get started with a Raspberry Pi, there are a number of operating system images already available. The one most people start with is Raspbian, which is based on Debian Linux.
Adafruit created a fork of Raspbian which they called Occidentalis. Their aim, of course, was for hardware hacking with the Pi. Included are some patches that help make accessing the breakout pins easier.
Their latest project is the Adafruit Pi Finder, which makes it easier to configure a Pi over a network. Check out the details on Adafruit’s blog.