Diff
Compares two texts by character, word or line: a longest-common-subsequence table (with the shared prefix and suffix trimmed first) marks insertions and deletions inline, with a similarity percentage and an edit distance that counts insertions and deletions, not substitutions. The longest common subsequence is also the core of Unix diff, described by J. W. Hunt and M. D. McIlroy (Bell Labs, 1976).