Dana Ross

This plugin displays a message at the top of the screen for visitors running outdated versions of the most popular browsers:
<ul>
<li>Internet Explorer 6 SP2 or less</li>
<li>FireFox 2 or less</li>
<li>Safari 2 or less</li>
</ul>
It is designed to be unobtrusive. The message disappears after a few seconds, and it only displays the first time the visitor comes to your WordPress blog/site.
The warning reads: It appears that you are using an outdated version of your browser. Please upgrade your browser for better sercurity and an improved experience.
The message also includes a link where the newest version of the browser can be downloaded.

Dave’s Outdated Browser Warning

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 …

Dave’s Outdated Browser Warning vs Browser Specific CSS