Haris Zulfiqar

WordPress tooltip plugins lets you add tooltips to plain text and links on your post and pages.
It offers a tinyMCE shortcode GUI which will guide you through the entire process of a tooltip.
This plugin uses jQuery tipTip authored by Drew Wilson http://code.drewwilson.com/entry/tiptip-jquery-plugin.
You can also add tooltips manually by following the pattern below:
<pre><code>[tooltip content="This will show in a tooltip" url="This will make tooltip a link"]Hover over me for tooltip[/tooltip]
</code></pre>
Url is optional. If URL is added, it will render a link otherwise it will be a plain text.

WordPress Tooltip

WordPress tooltip lets you add tooltips to content on your posts and pages.

Felix W.

Based on the tooltip collection from http://tympanus.net/codrops/2014/10/07/tooltip-styles-inspiration/ this plugin provides different tooltip styles.
You can easily add tooltips by using the small form displayed in the TinyMCE or you manually use the shortcodes.

Tooltip Crazy

Tooltip Crazy adds a new button to the TinyMCE for adding tooltips to your posts and sites via shortcode.

WordPress Tooltip vs Tooltip Crazy