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.

Japh

Plugin for WordPress to insert the code snippet for <a href="http://IE6Update.com" rel="nofollow ugc">IE6Update</a> into your WordPress pages. 
Read more about the IE6Update code snippet here: <a href="http://almost.done21.com/2009/04/announcing-ie6-update-help-kill-internet-explorer-6/" rel="nofollow ugc">Announcing IE6 Update – Help Kill Internet Explorer 6</a>

WP-IE6Update

Plugin for WordPress to insert the code snippet for IE6Update into your WordPress pages.

Block IE6 vs WP-IE6Update