Richard Bush

The 42U HTML5 Upgrade prepares your older site for HTML5 and allows you to start using HTML5 structure tags right now in your posts and pages (section, aside, article, etc), regardless of the browser viewing them.
The 42U HTML5 Upgrade plugin loads <a href="http://www.modernizr.com/" title="Modernizr" rel="nofollow ugc">Modernizr</a>, enables the HTML5 Schema in the <a href="http://www.tinymce.com/" title="TinyMCE" rel="nofollow ugc">TinyMCE Editor</a> and adds HTML5 elements to the TinyMCE style list.
The custom Modernizr build includes all CSS3, HTML5 and Misc tests, as well as the html5shiv w/printshiv and Modernizr.load(). See the options we used <a href="http://www.42umbrellas.com/files/2009/06/modernizer2.6.1build.png" title="here" rel="nofollow ugc">here</a>.
You can turn these functions off individually if you wish!

42U HTML5 Upgrade

The 42U HTML5 Upgrade prepares your older site for HTML5 and allows you to start using HTML5 structure tags right now.

Ramoonus

This plugin adds the Modernizr to your WordPress installation. 
Modernizr is a small JavaScript library that detects the availability of native implementations for next-generation web technologies, i.e. features that stem from the HTML5 and CSS3 specifications. Many of these features are already implemented in at least one major browser (most of them in two or more), and what Modernizr does is, very simply, tell you whether the current browser has this feature natively implemented or not.
For documentation visit http://www.modernizr.com/docs/

Modernizr for WordPress

The 2.8.4 version inserts broken javascript link. I posted a message about this in the support section for the plugin and the author has not responded or fixed the problem. Now, any site that I've added this plugin to can be broken if the user runs an update. 

Instead, if you need Modernizr, insert the following into your header.php file just after <?php wp_head(); ?>:

<script type="text/javascript" src="https://cdnjs.cloudflare.com/ajax/libs/modernizr/2.8.3/modernizr.min.js"></script>

Doesn't work - Plugin Defunct

This plugin adds the Modernizr to your WordPress installation.

42U HTML5 Upgrade vs Modernizr for WordPress