CriticMarkup

Criticmarkup is a specialized markup language "for authors and editors to track changes to documents in plain text. As with Markdown, small groups of distinctive characters allow you to highlight insertions, deletions, substitutions and comments, all without the overhead of heavy, proprietary office suites." It is specially designed to work with "Markdown, MultiMarkdown and HTML, but it's compatible with most plain text formatting conventions."

It has five types of markup:

1. Addition {++ ++}
2. Deletion {-- --}
3. Substitution {~~ ~> ~~}
4. Comment {>> <<}
5. Highlight {{ }}{>> <<}

It's an interesting idea. I think I will wait with implementing until it is integrated into MultiMarkdown Composer.

Another issue is, of course, that the person with whom you collaborate also needs to use it.

See also the specs and this account.