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Wills are an especially rich source of names, which is a great way to jumpstart cluster or FAN (friends, associates, neighbors) research. Even something as simple as an entry in the National Probate Index can provide important clues.
This answer became Weekly Topic Challenge #6: http://meta.genealogy.stackexchange.com/questions/1926/learning-more-from-probate-records-and-willsMaking the most of clues from Probate Records and Wills?