'Free software' here not means 'libre software'.
Most similar to Gaussian, and code is really clean and easy to read, even have documents for developers.
An Open-Source electronic structure program emphasizing automation, advanced libraries, and interoperability.
Has potentials for solid-state materials (metals, semiconductors) and soft matter (biomolecules, polymers) and coarse-grained or mesoscopic systems. It can be used to model atoms or, more generically, as a parallel particle simulator at the atomic, meso, or continuum scale.
Specializes in periodic systems with plane wave basis sets.
Specializes in high level quantum chemistry calculations.
Taken the best features of parallel implementations of quantum chemistry methods for electronic structure.
Oriented towards relativistic quantum chemistry problems.
Can calculate a smaller set of properties but it can handle mixed QM/MM calculations and periodic systems like solids.