Spin off of the original answer to the linked question.
- Grammar Check: I am not aware of any dedicated grammar parsers for emacs yet. I will mention
writegood-modeavailable in MELPA which highlights weasel words and passive voice in the buffer. It gets you half the way there.
Another possibly useful library is dupwords.el which can highlight if a particular word (typically nouns and pronouns) are used more than once in the same sentence. This is more general than finding repeated adjacent words which can be handled by flyspell.
- Anything else: Correcting language in general is a hard problem. That being said, if there exists any external program/script in this area, it is usually easy to make it interface with emacs.
If you use LaTeX for writing, I have personally found the following ruby scripts quite useful for checking style and consistency. I just call the shell command from the minibuffer itself. A viewer capable of forward and inverse search is also valuable when proofreading. On MS Windows, I use sumatra-forward package from ELPA for forward search in SumatraPDF viewer.
