Myrna Larson and Bill Manville have developed a compare that's very nice.

http://www.cpearson.com/excel/whatsnew.htm
look for compare.xla

But the bad news is that this does a cell-by-cell comparison.  A1 compares to
A1, x99 to x99, etc.

If you insert/delete a row or column, then this won't work very well.

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Other alternatives that may work depending on what kind of differences you're
looking for:

Save each worksheet as a .csv file and use any comparison program you want to
compare two text files--like MSWord.