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.

Pierre-André Boissinot

Semio Design

This plugin detects if a visitor is using some old version of Internet Explorer and suggests to upgrade to a recent browser. 
The versions to block can be configured in an admin panel. 
Works for blocking IE6, IE7 and IE8. 
Localisation of the plugin into french.

Stop Oldies

This plugin detects if a visitor is using some old version of Internet Explorer and suggests to upgrade to a recent browser.

Modernizr for WordPress vs Stop Oldies