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:
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