Is actually an operating system installer, not an OS itself.
A Debian derivative. Available as a normal version (4 GB card needed) and Lite version (512 MB).
Based on Ubuntu.
Lightweight Linux distribution. Images are no longer maintained for Raspberry. Installation is done from tarballs now with manual setup of partitions and file systems, see here.
Fedora based. No longer maintained. For Raspberry Pi 1 only.
Fedora based. Raspberry 2 and 3.
A small operating system focusing on network connectivity.
openwrt-brcm2708-bcm2708-sdcard-vfat-ext4.img for Raspberry Pi 1.
openwrt-brcm2708-bcm2709-sdcard-vfat-ext4.img for Raspberry Pi 2/3
OSMC (formerly RaspBMC)
Open source media center.
Windows 10 IoT Core
Can run Universal .NET Core apps.
Turns your Pi into an arcade station.
Download (2 GB card needed)
OS running with small CPU and memory footprint. Is not a Linux derivative.
Experimental Unix clone. Read the Newbie Guide, e.g. to know that the Del key kills programs.
Download for Pi 1, Download for Pi 2
Based on FreeBSD 11.
Download (16 MB card min, 1GB recommended)
For embedded systems, a robust in-RAM only distro.