The open source version of Zotonic can be downloaded from GitHub.
Get the latest release
Download the latest release of Zotonic from Github as a .zip file.
The latest release of Zotonic is release 0.42. Read the release notes.
To run the latest-and-greatest development version of Zotonic, you can check out your own local copy of the Zotonic repository with:
git clone git://github.com/zotonic/zotonic.git
This will make a complete copy of the "master" branch of the Zotonic repository. Github provides a nice browsable overview of Zotonic's source code and the change logs.
Please check the installation guide for required third party software, build process and configuration.