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.

lupomontero

This plugin replaces the contents of your site with a message asking your 
visitors to use a better browser if they try to browse your site with Internet 
Explorer 7 or earlier.

No IE

'No IE' blocks visits from Internet Explorer 7 and earlier and displays a nice message with a list of better browsers.

Block IE6 vs No IE