RPi OSW-Picked

You've picked an OS!

What's Linux?
As you don't know what Linux is, it's preferable to skip over to the Raspbian section as thats the canonical Linux version used on the PI (but Linux isn't the only choice).

IWannaWindows
Oh, I see how it is. Windows only supports x86 CPU architectures, so if you want to port Windows onto ARM, Windows RT won't work, so consider editing ReactOS and make it ARM compatible. (If you know what to do, that is. :) It would be awesome to the community!)

But do understand that even if ReactOS itself would work, you could run none of the x86 Windows applications on it.

Raspbian
Hello, newcomer (to Linux and/or the Raspberry Pi). To get Raspbian, the official OS for the Raspberry Pi, click here. Raspbian is a specialized version of Linux just for the Raspberry Pi!

Nobuntu
Because of Canonical's stubborn decision, they have decided to not support ARM 6 and below. We'd recommend either Raspbian or Debian, as Ubuntu is a derivative of Debian.

Fedora or Red Hat
Shoot! Red Hat is not making an ARM version available! Please try the default distro.

ArchLinux
Hmm, congrats! Arch Linux IS supported by the RasPi Foundation! Please click here for an Arch Linux download for the RasPi.

Debian
Ok. Good for you. Please click here for an appropriate Debian download for the RasPi.

Puppy linux
Often regarded as the ultimate Linux distro for "small systems", there is now an experimental version of Puppy Linux for the PI called "Squeezed Arm Puppy". Its especially suitable for people who never used anything but Windows, as its made to somewhat resemble and work like windows, but its still Linux. It comes with many extras.

AROS
AROS research operating System. A remake of the old Amiga OS

Android
Googles OS for mobile systems, is in development in the form of Android 4.0 ("Ice Cream Sandwich"). It's not out yet.

Firefox OS
An OS based on the code behind the browser Firefox, also in development.

WebOS
Based on the former OS for Palm, later acquired by HP, in the form of "OpenWebOS" an open source version is in development

Plan 9 from Bell Labs
Is an very experimental OS, which is loosely based on the original Unix taken to its extremes, only suitable (written for) programmers, at this time.

RISC OS
easily the canonical OS for ARM based computers, first developed for the Acorn Archimedes, the computer that should be te successor of the BBC Beeb, the computer that inspired the PI (its why the two versions of the PI are called the "model A" and "Model B", just like the Beeb from yesteryears. Very fast and smooth running on the PI.

BSD
Berkley Software Distribution. Although not legally Unix FreeBSD and NETBSD are two very Unix like OS's which are well established.

Others
Not exactly OS's in a technical sense, but there are also specific software sets that can be put on a SD-Card for specific applications. Just to mention a few, there is XMBC which is a dedicated media player, and Instant Kiosk which is an instant ethernet browser (kiosk) system.

Other similar packages are IPFire, OpenELEC, Raspbmc, XBMC and XBian