Scott Reilly

Even though you may add two spaces after each sentence when writing a post (assuming you subscribe to a writing style that suggests such spacing) web browsers will collapse consecutive blank spaces into a single space when viewed. This plugin adds a <code>&nbsp;</code> (non-breaking space) after sentence-ending punctuation to retain the appearance of your two-space intent.
NOTE: The plugin will only enforce the two-space gap in places where two or more spaces actually separate sentences in your posts. It will NOT insert a second space if only one space is present.
Links: <a href="http://coffee2code.com/wp-plugins/extra-sentence-space/" rel="nofollow ugc">Plugin Homepage</a> | <a href="https://wordpress.org/plugins/extra-sentence-space/" rel="ugc">Plugin Directory Page</a> | <a href="https://github.com/coffee2code/extra-sentence-space/" rel="nofollow ugc">GitHub</a> | <a href="http://coffee2code.com" rel="nofollow ugc">Author Homepage</a>
<h3>Hooks</h3>
The plugin is further customizable via two filters. Typically, these customizations would be put into your active theme’s functions.php file, or used by another plugin.
c2c_extra_sentence_space
The ‘c2c_extra_sentence_space’ filter allows you to use an alternative approach to safely invoke <code>c2c_extra_sentence_space()</code> in such a way that if the plugin were deactivated or deleted, then your calls to the function won’t cause errors in your site. This only applies if you use the function directly, which is not typical usage for most users.
Example:
Instead of:
<pre><code><?php echo c2c_extra_sentence_space( $mytext ); ?>
</code></pre>
Do:
<pre><code><?php echo apply_filters( 'c2c_extra_sentence_space', $mytext ); ?>
</code></pre>
c2c_extra_sentence_space_punctuation
The ‘c2c_extra_sentence_space_punctuation’ filter allows you to customize the punctuation, characters, and/or symbols after which double-spacing (when present) is preserved. By default these are ‘.!?’.
Arguments:
<ul>
<li>$punctuation (string): The default characters after which double-spacing should be preserved. Default is ‘.!?’.</li>
</ul>
Example:
<pre><code>/**
 * Modifies the list of characters after which two spaces should be preserved
 * to include a forward slash.
 *
 * @param string $punctuation The punctuation.
 * @return string
 */
function more_extra_space_punctuation( $punctuation ) {
 // Add the '/' and ')' characters to the list of characters
 return $punctuation . '/)';
}
add_filter( 'c2c_extra_sentence_space_punctuation', 'more_extra_space_punctuation' );
</code></pre>

Extra Sentence Space

Excellent

Force browsers to display two spaces (when present) between sentences.

LastLeaf

This plugin helps you to keep the blanks in visual editor (TinyMCE). 
If you start paragraphs with spaces, you will find this plugin helpful. 
The code of this plugin is modified from stringfold’s TinyMCE Entities Patch, which is no longer updated.
<h3>Arbitrary section</h3>

Keep Blanks

This plugin helps you to keep the blanks in visual editor (TinyMCE).

Extra Sentence Space vs Keep Blanks