Coot

Coot is for macromolecular model building, model completion and validation, particularly suitable for protein modelling using X-ray data.

Coot displays maps and models and allows model manipulations such as idealization, real space refinement, manual rotation/translation, rigid-body fitting, ligand search, solvation, mutations, rotamers, Ramachandran plots, skeletonization, non-crystallographic symmetry and more.

http://www.biop.ox.ac.uk/coot/