Mistape allows readers to effortlessly notify site staff about found spelling errors.

The WordPress E-Prime Grammar Checker improves your writing style by showing hints on to how avoid the passive voice and improve clarity.

Implements After The Deadline proofreading on the comments form.

TinyMCE Spellcheck adds the spellcheck button back to the editor in WordPress 3.6 and up.

ReWord - Make It Right! Allow your readers to help and report mistakes in your site.