We are pleased to announce the fifteenth release (code name "Hack") of the Einstein Toolkit, an open, community developed software infrastructure for relativistic astrophysics. The highlights of this release are: * The new Piraha parser now parses all CCL and parameter files. The only user-visible change should be improved error messages. * A much more general, piece-wise EoS for EOS_Omni and the corresponding improvements for neutron star initial data using the Lorene library In addition, bug fixes accumulated since the previous release in December 2016 have been included. The Einstein Toolkit is a collection of software components and tools for simulating and analyzing general relativistic astrophysical systems that builds on numerous software efforts in the numerical relativity community including CactusEinstein, the Carpet AMR infrastructure and the relativistic magneto-hydrodynamics code GRHydro. For parts of the toolkit, the Cactus Framework is used as the underlying computational infrastructure providing large-scale parallelization, general computational components, and a model for collaborative, portable code development. The toolkit includes modules to build complete codes for simulating black hole spacetimes as well as systems governed by relativistic magneto-hydrodynamics. The Einstein Toolkit uses a distributed software model and its different modules are developed, distributed, and supported either by the core team of Einstein Toolkit Maintainers, or by individual groups. Where modules are provided by external groups, the Einstein Toolkit Maintainers provide quality control for modules for inclusion in the toolkit and help coordinate support. The Einstein Toolkit Maintainers currently involve postdocs and faculty from six different institutions, and host weekly meetings that are open for anyone to join in. Guiding principles for the design and implementation of the toolkit include: open, community-driven software development; well thought out and stable interfaces; separation of physics software from computational science infrastructure; provision of complete working production code; training and education for a new generation of researchers. For more information about using or contributing to the Einstein Toolkit, or to join the Einstein Toolkit Consortium, please visit our web pages at http://einsteintoolkit.org. The Einstein Toolkit is primarily supported by NSF 1550551/1550461/1550436/1550514 (Einstein Toolkit Community Integration and Data Exploration). The Einstein Toolkit contains about 200 regression test cases. On a large portion of the tested machines, almost all of these test suites pass, using both MPI and OpenMP parallelization.