Google Code hosts a library written by Neil Fraser, called google-diff-match-patch.
The Diff Match and Patch libraries offer robust algorithms to perform the operations required for synchronizing plain text.
- Diff: Compare two blocks of plain text and efficiently return a list of differences.
- Match: Given a search string, find its best fuzzy match in a block of plain text. Weighted for both accuracy and location.
- Patch: Apply a list of patches onto plain text. Use best-effort to apply patch even when the underlying text doesn’t match.
Google Documents uses this library. Reasonable content management systems should use it, too.
More information and download at Google Code: diff-match-patch.