The reference implementation of Org is part of Emacs. If you already have Emacs installed, you can start using org-mode right away!

You can install the latest version of Org as a package by invoking M-x package-install RET org RET once you have configured Emacs to include Org ELPA as a package repository.

Alternatively you can download the package as a tar.gz or zip archive.

The bleeding-edge development version or Org is available at code.orgmode.org. You can clone the repo via:

~$ git clone https://code.orgmode.org/bzg/org-mode.git

Daily snapshots of the development version are released as tar.gz or zip files.

Worg has documentation about build options and about how to stay on the bleeding edge.

There are also many packages that augment Org available on MELPA.