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

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.

Gecka IE Warning vs Compatiblizr