For the Raspberry Pi (version 1 and 2) there are multiple operating systems available. Downloads of each operating system can be found at www.raspberrypi.org/downloads/

Raspbian

Raspbian is the preffered Raspberry Pi operating system and mostly used by others. It`s based on Debian and features official firmware and tools.

I recommend Raspbian

Snappy Ubuntu Core

Snappy Ubuntu Core is a relatively new operating system. It`s based on Ubuntu Linux but with extra features. This might be the next big thing!

OpenElec

OpenElec is the prefferd operating system when you want to turn your raspberry pi in a HTPC with XMBC software.

Pidora

Pidora is Fedora Linux for the Raspberry Pi. It`s a bit different from Raspbian and really interesting if you are familiar with Fedora/Red Hat Linux.

Risc OS

RISC OS is unlike the other distributions. More information about RISC OS for the Raspberry Pi. I have no idea why you would run this operating system.