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.

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.

Block IE6 vs Stop Oldies