Topas is a powerful suite of software written by Alan Coelho for the analysis of powder diffraction data. Bruker (http://www.bruker-axs.de/) market the original version of topas which comes with a full graphical user interface. An academic-only version is also available from Alan Coelho (http://www.topas-academic.net/).

Topas allows you to:

• analyse constant wavelength and energy dispersive X-ray data
• analyse constant wavelength and time of flight neutron data
• perform Rietveld/Pawley/individual reflection fitting
• index powder diffraction data
• solve structures by simulated annealing and charge flipping
• perform isodisplace distortion mode refinements
• etc

You can find lots more information about topas on various web sites. Try:

John Evans' site: http://www.dur.ac.uk/john.evans/topas_academic/topas_main.htm
Alan Coelho's site: http://www.topas-academic.net/
Bruker's site: http://www.bruker-axs.de/topas.html