The most popular and stable standalone distribution at the moment is
Raspbian Wheezy (Debian) Raspbian Jessie – this will give you a bit more control over installation , you can download all the software here. If you’re a Linux user you will be familiar with LX Terminal – this is what we will be using to write the image to the SD Card. Open up LX Terminal and type in the following code to identify the SD Card:
We’re looking for something like “/dev/sdd1” and we need to ensure its the whole volume – not just a partition. Unmount the card with the following command:
sudo umount /dev/sdd1
Locate the folder where you have downloaded the .img and make a not of the hierarchy (i.e. Home/Downloads//2012-12-16-wheezy-raspbian.img). Write the image to the Sd Card with the following command:
sudo dd bs=4M if=~/2012-12-16-wheezy-raspbian.img of=/dev/sdd1
There is no progress indicator here, but give it 15-20 mins , have a coffee break and come back to a freshly baked Pi Card. Eject card when finished, insert into Raspberry pi and boot up.
UPDATE; Go here for the EASY Raspbian OS install procedure.