CP2K is a program to perform atomistic and molecular simulations of solid state, liquid, molecular, and biological systems. It provides a general framework for different methods such as e.g., density functional theory (DFT) using a mixed Gaussian and plane waves approach (GPW) and classical pair and many-body potentials.
CP2K is freely available under the GPL license. It is written in Fortran 2003 and can be run efficiently in parallel.
CP2K provides state-of-the-art methods for efficient and accurate atomistic simulations, sources are freely available and actively improved. It is therefore easy to give the code a try, and to make modifications as needed. However, CP2K comes without any warranty or direct support.
A partial answer is given by the list of developers having SVN write permission.