Gecka

Display a configurable warning for Internet Explorer upgrade.
Choose to display the warning for versions before IE8 or before IE7.
Choose to always display the warning, or once per session, or once a day, or every 30 days or every year.
The plugin configuration appears in the Reading Settings page.
(based on <a href="http://code.google.com/p/ie6-upgrade-warning/" rel="nofollow ugc">ie6-upgrade-warning</a>)

Gecka IE Warning

Display a configurable warning for some versions of Internet Explorer

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.

Gecka IE Warning vs Stop Oldies