TinyMCE Spellcheck is new WordPress plugin that lets you add a simple spellcheck button in visual editor present in WordPress add new & edit post pages. The plugin is easy to use and automatically starts working after activation. It is a fork of After the Deadline plugin which is now a part of Jetpack by WordPress.Com plugin.
TinyMCE Spellcheck in WordPress Visual editor
An example of TinyMCE Spellcheck behavior
Installation & Usage: Install and activate TinyMCE Spellcheck. Upon activation the plugin adds a Proofread Writing button in visual editor present in content editing admin pages.