Thomas

Really simple plugin that allows you to easily add tooltips to your posts using the shortcode [wpetp].
It was made for some simple tooltips with text and/or images. For example if you want a way to add some information about a word in your post without forcing the user to leave the page to see them.
Feel free to ask for some features if you think I missed some.
Languages: English.

WP Easy Tooltips

Simple plugin that allows you to easily add tooltips to your posts using a shortcode.

techotronic

Adds tinytips tooltips to links on your site. Comes with 8 different themes. 
The tinytip can be added to a link using the WordPress visual editor.
The plugin can be automated to add the functionality for all links.
Tinytips tooltips are added for all links that have a “title” attribute.
See the <a href="http://www.techotronic.de/plugins/jquery-tinytips/" rel="nofollow ugc">plugin page</a> for demo pages.
For more information visit the <a href="https://wordpress.org/extend/plugins/jquery-tinytips/faq/" rel="ugc">FAQ</a>. 
If you have questions or problems, feel free to write an email to blog [at] techotronic.de or write an entry at <a href="https://wordpress.org/tags/jquery-tinytips?forum_id=10" rel="ugc">the jQuery Tinytips WordPress.org forum</a>
Localization
<ul>
<li>English (en_EN) by <a href="http://www.techotronic.de/" rel="nofollow ugc">Arne Franken</a></li>
<li>German (de_DE) by <a href="http://www.techotronic.de/" rel="nofollow ugc">Arne Franken</a></li>
</ul>
Includes <a href="http://plugins.jquery.com/project/tinytips" rel="nofollow ugc">Tinytips</a> 1.1 jQuery plugin from <a href="http://www.mikemerritt.me/" rel="nofollow ugc">Mike Merritt</a>. 
Tinytips is licensed as <a href="http://www.mikemerritt.me/plugins/tinyTips/license.txt" rel="nofollow ugc">free of charge for anyone</a>. 
jQuery Colorbox uses the jQuery library version 1.4.2 bundled with WordPress 3.0.

jQuery Tinytips

Adds tinytips tooltips to links on your site. Comes with different themes.

WP Easy Tooltips vs jQuery Tinytips