I am looking for a program (preferably a web app like Google Docs) that enables lightweight commenting / review / annotation of textual content by multiple users.
I want something rather like Google Documents or Amazon Kindle's annotations, but preferably geared towards plain text. The ability to just 'tag' a specific word, sentence or paragraph would be helpful.
This would be for written content, but may be helpful for code too. Perhaps a lightweight enough code review tool would work for both text and code.
If such a tool supported tracking changes ('diffs') that would be helpful too.