Felipe Lavín

This plugin adds an editor button that will let you use the great <a href="http://codemirror.net" rel="nofollow ugc">CodeMirror</a> editor to edit the HTML code of your posts.
This editor supports code coloring and automatic tag closing on a big editor that will let you write beautiful code on your WordPress posts.
Probably the best choice if you need to use complex HTML on your posts, or just enjoy writing in plain HTML.

CodeMirror for Post Editor

Write beautiful HTML code using the CodeMirror component for your WordPress posts.

Matthew Muro

TinyMCE Spellcheck adds the spellcheck button back to the editor in WordPress 3.6 and up. This is a fork of the After the Deadline plugin which is now a part of Jetpack.

TinyMCE Spellcheck

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

CodeMirror for Post Editor vs TinyMCE Spellcheck