Force Field X is a group of open source (GPL v. 3), platform independent (Java Runtime Environment) modules for molecular biophysics. Key methods include:

Polarizable AMOEBA force fields
Particle-mesh Ewald electrostatics
Generalized Kirkwood continuum electrostatics
X-ray and neutron crystallography refinement
Real space refinement for CryoEM
Methods for structure based drug design

for more information, see http://ffx.kenai.com