Starting in the Spring of 2016, with the 4.0 version, SIESTA is released under the terms of the GPL open-source license. Source packages and access to the development versions can be obtained from the Gitlab site.
For an overview of the various strands of development, please see this Guide to SIESTA versions.
Other versions of SIESTA are distributed free of charge to academics (please see the detailed conditions in the licence text), and can be installed by computer centers to be used only by academics. These are not open source versions: they cannot be redistributed. They can be accessed here.