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.

Designer023

IE 7 and 8 don’t do CSS3 or Media queries so instead of adding the fixes to your theme you can add them via a plugin. This means when they finally die a death you can drop support without having to update your theme. This also covers purchased themes so you don’t need to alter the actual files.

Compatiblizr

Plugin for patching old versions of IE (7 and 8)to work with CSS3 selectors and Media Queries.

Stop Oldies vs Compatiblizr