Adrian Hanft

The Browser Specific CSS plugin is a tool that allows developers to easily target different browsers straight from the stylesheet. Browser Specific CSS adds a short javascript to the head of your page that enables you to use browser specific css selectors. For example, targeting Internet Explorer 7 from your stylesheet is just a matter of defining styles with a “.ie7” selector. Every major browser is supported including code for targeting Macs, PCs, and Linux operating systems.
This plugin uses the javascript written by <a href="http://rafael.adm.br/css_browser_selector/" title="Rafael Lima" rel="nofollow ugc">Rafael Lima</a>. No changes have been made to his code, and all the credit goes to him. Thanks, Rafael!
Changelog: 
Version 0.3 
– Update for compatibility with WordPress 2.9
Version 0.2 
– Corrected readme file.
Version 0.1 
– Features of the first version of this plugin include the ability to turn the plugin on and off and to specify a separate stylesheet.

Browser Specific CSS

The Browser Specific CSS Plugin allows you to easily target specific browsers and operating systems from your theme's stylesheet using regular cs …

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.

Browser Specific CSS vs Stop Oldies