I am not aware of a free download, but

http://groups.google.com/group/micro...f1ceadb78ba375

shows how to do the Wilcoxon test (equivalent to Mann-Whitney) in Excel.
You might also find

http://en.wikipedia.org/wiki/Mann-whitney

to be instructive


Commercial add-ins are available, such as those from

www.analyse-it.com/
www.xlstat.com
www.palisade.com/

(I have no personal experiance with any of them)


Outside of Excel, if you don't mind learning the S language, you could use
the freely available open-source package R

www.r-project.org


Jerry