Vast HTML

This plugin keeps people from viewing your site if they are using Internet Explorer 6. Gives options to download a variety of newer browsers.
For more information on the plugin visit: http://vasthtml.com/js/wordpress-block-ie6-plugin/

Block IE6

This Wordpress plugin will block IE6 users with a choice to upgrade.

Adrian Hanft

The Browser Specific CSS plugin is a tool that allows developers to easily target different browsers straight from the stylesheet. Browser Specific CSS adds a short javascript to the head of your page that enables you to use browser specific css selectors. For example, targeting Internet Explorer 7 from your stylesheet is just a matter of defining styles with a “.ie7” selector. Every major browser is supported including code for targeting Macs, PCs, and Linux operating systems.
This plugin uses the javascript written by <a href="http://rafael.adm.br/css_browser_selector/" title="Rafael Lima" rel="nofollow ugc">Rafael Lima</a>. No changes have been made to his code, and all the credit goes to him. Thanks, Rafael!
Changelog: 
Version 0.3 
– Update for compatibility with WordPress 2.9
Version 0.2 
– Corrected readme file.
Version 0.1 
– Features of the first version of this plugin include the ability to turn the plugin on and off and to specify a separate stylesheet.

Browser Specific CSS

The Browser Specific CSS Plugin allows you to easily target specific browsers and operating systems from your theme's stylesheet using regular cs …

Block IE6 vs Browser Specific CSS