MCMS ships with a nice comparison feature for revisions. But there is nothing available to compare to different postings!
Chester has spent some time to analyze how MCMS does the revision history comparison and managed to get the CompareHTML.dll shipped with MCMS and used for revision history comparision working in his implementation to compare different postings:
http://chestermr.blogspot.com/2005/09/using-comparehtml-to-compare-postings.html