PowderCell powder pattern calculation (by Werner Kraus and Gert Nolze) (http://www.ccp14.ac.uk/tutorial/powdcell/)
Can model multiple phases for quantitative analysis or modeling impurities
Interactively modify atom occupancies and observe changes in the powder pattern
Move atoms using point and click then see the change in the powder pattern
Overlay a calculated powder pattern with raw diffraction data.
Whole pattern unit cell refinement