TinyMCE SpellChecker API – this plugin provides action/filter hooks to allow easy customisation of the TinyMCE SpellChecker.
This plugin was created to solve the problem of not being able to use GoogleSpell behind a firewall. For background information on the issue read the discussions on <a href="https://core.trac.wordpress.org/ticket/9798" rel="nofollow ugc">trac ticket #9798</a>. The ultimate goal of this plugin is to be merged into WordPress Core.
You need this plugin if…
<ul>
<li>You want to write a plugin for the TinyMCE SpellChecker. This plugin will provide you with respective hooks that allow to control which SpellChecker Engine is used, adjust the configuration used for initialisation of the engine and manipulate spellchecking results (independent of engine).</li>
<li>You are behind a firewall and you’d like to use the Google Spellchecker. This plugin contains a customised version of GoogleSpell that uses the new Http Transportation methods added in WordPress 2.8 which include Proxy Support for all external requests.</li>
</ul>
This plugin also contains a simple sub-plugin called “WP-SpellCheck Exceptions” which allows you to add custom words to the spellchecking dictionary. Note that this plugin is more a proof of concept than a complete/full-fleged solution. (It does not have any kind of visual administration; it only adds “WordPress” as an exception at the moment).

WP-Spellcheck

TinyMCE SpellChecker API - this plugin provides action/filter hooks to allow easy customisation of the TinyMCE SpellChecker.

TiomKing

ReWord WordPress plugin allows the users to suggest fixes for content mistakes on a wordpress site. It enhances users experience, and lets the readers take part in editing and proofreading content, while providing a reliable online content checking, using the power of the masses.
Intuitive frontend User Interface to report mistakes and send them to Administrator. Just mark mistake text, click on “R” icon, add your fix and send it.
The reports admin page displays all reported mistakes, and lets admin fix them, or ignore them. Admin can also set the number of alerts before showing a report, to ensure accurate reports and real issues detection in site content.

ReWord

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

WP-Spellcheck vs ReWord