Adafruit Raspberry Pi Educational Linux Distro
Adafruit's tweaked distribution for teaching electronics using the Raspberry Pi.
Gentoo is a custom Gentoo overlay for use with the Rpi2.
With Pi MusicBox, you can create a cheap (Sonos-like) standalone streaming music player for Spotify, Google Music, SoundCloud, Webradio, Podcasts and other music from the cloud.
PressPi is a turn key Raspberry Pi Wordpress server. PressPi allows you to have a fully configured WordPress server up and running in just minutes.
Raspbian is a free operating system based on Debian optimized for the Raspberry Pi hardware. An operating system is the set of basic programs and utilities that make your Raspberry Pi run. However, Raspbian provides more than a pure OS: it comes with over 35,000 packages, pre-compiled software bundled in a nice format for easy installation on your Raspberry Pi.
RetroPie bundles together more than 30 emulators of much-loved systems from the past 30 years - including Amigas, Ataris, the N64 and the Sega Mega Drive.
RiscOS is a fast and easily customised operating system for ARM devices.