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 95 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.
CP2K comes without any warranty and relatively little, but growing, documentation. Using CP2K for production simulations requires a good understanding of the underlying theory and algorithms.
Several releases have been made (Download Here). Active development takes place in the SVN trunk, where changes, improvements and bug fixes are made at (ir)regular intervals.
A partial answer is given by the list of developers having SVN write permission.