Kevin Newman

WraithKenny

This plugin allows Admin users the ability to add custom CSS and JavaScript directly into individual Post, Pages or any other registered custom post types. You can also add classes to the body tag and the post container. There is a Global settings page for which you can write Scripts n Styles for the entire blog.
Admin’s can also add classes to the TinyMCE “Formats” dropdown which users can use to style posts and pages directly. As of Scripts n Styles 3+ styles are reflected in the post editor.
Because only well trusted users should ever be allowed to insert JavaScript directly into the pages of your site, this plugin restricts usage to admin type users. Admin’s have access to even more sensitive areas by definition, so that should be relatively safe 😉
<h4>Notes about the implementation:</h4>
<ul>
<li>Admin users, or more specifically, any user with the <code>manage_options</code> and <code>unfiltered_html</code> capabilities (which by default is only the admin type user) can use this plugin’s functionality. Some plugins extend user rolls, and so this plugin would naturally extend include rolls that have the appropriate capability.</li>
<li>CSS Styles are embeded, not linked, at the bottom of the <code>head</code> element with <code>style</code> tags by using <code>wp-head</code>. If your theme doesn’t have this hook, this plugin (as well as most others) won’t work.</li>
<li>JavaScript is embeded, not linked, at the bottom of the <code>body</code> (or <code>head</code>) element with <code>script</code> tags by using <code>wp-footer</code> (or <code>wp-head</code>). If your theme doesn’t have this hook, this plugin (as well as most others) won’t work.</li>
<li>There is no input validation. This plugin puts exactly what you type in the meta box directly into the <code>html</code> with no error checking. You are an Admin, and we trust you to be carefull. Try not to break anything.</li>
<li>Do to the licensing of the libraries used, this plugin is released “GPL 3.0 or later” if you care about those things.</li>
</ul>

Scripts n Styles

This plugin is simply awesome. 
No other words to say. 

Must have! Great Job.

This plugin allows Admin users to individually add HTML, custom CSS, Classes and JavaScript directly to Post, Pages or any other custom post types.

salubrio

Intended users: Template Designers, Developer, Admins
If you wish to add any custom HTML to each page’s header, then this plugin is for you.
This is useful for verifying you are the owner of the website to services such as Mailchimp or Google. You can quickly add the verification codes to your page header without having to edit your site’s template.
In general, you can add custom CSS, a link to an external JavaScript file or something else. While it is generally recommended to create a child template if you’re going to make extensive, permanent changes to a template, there may be instances where a small change or two is needed that wouldn’t justify the creation of a child template—or your current template might not support child templates. You should nearly always avoid editing a template directly, because your changes will be lost when you next update the template.
This plugin is not affected by template changes.
<h3>Acknowledgements</h3>
Plugin Icon (CC BY 3.0) by <a href="https://www.iconfinder.com/denir" rel="nofollow ugc">DeniShop</a>

Add Code To Head

Add custom JavaScript/HTML/CSS codes to the page head without editing the template.

Scripts n Styles vs Add Code To Head