Octopus is a scientific program aimed at the ab initio virtual experimentation on a hopefully ever-increasing range of system types.
ORCA is a flexible, reasonably efficient and easy-to-use general purpose tool for quantum chemistry with specific emphasis on spectroscopic properties of open-shell molecules. It features a wide variety of standard quantum chemical methods ranging from semiempirical methods to DFT to correlated ab initio methods. It can also treat environmental and relativistic effects.
PSI4 is an ab initio quantum chemistry program.
Quantum Espresso is an integrated suite of Open-Source computer codes for electronic-structure calculations and materials modeling at the nanoscale. It is based on density-functional theory, plane waves, and pseudopotentials.
SIESTA (Spanish Initiative for Electronic Simulations with Thousands of Atoms) is a program for electronic structure molecular dynamics calculations with a localized basis set.