ERP5 is released as an open-source software under the GNU General Public License Version 2. You can download it for free, use it for free and build upon new software as long as you respect the terms of the license.
There is to ways to download ERP5. The first one is to install ERP5 on your laptop. You will then have access to its code. The second one is to download the code, or read it online on the repository.
Install ERP5
Unless you want to create a mirror of ERP5 Git repository, the best way to download ERP5 is by installing it on your laptop. ERP5 can run on any platform and on any OS. You must me interested in the following pages:
Download the Source Code
All ERP5's code is available on a public Git repository: http://git.erp5.org/gitweb/erp5.git.
The code inside the repository is changing everyday as developers are working on it.
You can navigate through the code with any standard web-browser.
If you wish to download the entire code on your laptop, it can be done thanks to the following Git command line:
$ git clone http://git.erp5.org/repos/erp5.git