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

Deblyn Prado

So, you’re tired of people visiting your modern website with an outdated browser and not doing anything about it. Maybe they aren’t “power” users, maybe it’s your auntie running a last century browser trying to see awesome CSS3 animations and transforms. Let the user know that’s an outdated browser, and advise them on a better one.
With this solution you can check if the user’s browser can handle your website. If not, it will show a cool looking notice advising the user to update the browser. It\’ll be up to him/her to decide if he upgrades or not. Don’t force the user!
This plugin is a version of <a href="http://outdatedbrowser.com/" rel="nofollow ugc">Outdated Browser Projetct</a> created by <a href="https://github.com/burocratik" rel="nofollow ugc">Bürocratik</a>

WP Outdated Browser

Show a message if the user using a outdated browser.

Dave’s Outdated Browser Warning vs WP Outdated Browser