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.

Vinnie

This plugin includes the <code>unitpngfix.js</code> javascript file if the browser is IE6 or lower. In plain words, it implements the solution for the PNG trasparency provided by Unit Interactive Labs (http://labs.unitinteractive.com/unitpngfix.php). It works on img objects and background-image attributes.
<h3>License</h3>
This program is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 3 of the License, or (at your option) any later version. 
This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Library General Public License for more details. 
You should have received a copy of the GNU General Public License along with this program; if not, write to the Free Software Foundation, Inc., 59 Temple Place – Suite 330, Boston, MA 02111-1307, USA.

Wp-UnitPNGfix

This plugin includes the unitpngfix.js javascript file if the browser is IE6 or lower. In plain words, it implements the solution for the PNG traspare …

Block IE6 vs Wp-UnitPNGfix