Yarick

This plugin notifies user if browser check failed. Available checks:
<ul>
<li>Browser version (Chrome, Firefox, Safari, Opera, IE)</li>
<li>JavaScript enable</li>
<li>Cookie enable (require JavaScript)</li>
<li>Adobe Flash Player version (require JavaScript)</li>
</ul>
You can select style for notice. Or disable plugin CSS and write own.
User can close notice to time you set in minutes. It use cookie. You can disable close button.
Also you may set next check time in days. When check is successful.
Browser Check may be integrated with theme. Just place this code: “do_action(‘browser_check’)” in your theme. And disable plugin css from “Settings -> Browser Check” page and write own css for best look in your theme.
Supported languages
<ul>
<li>English</li>
<li>Russian</li>
</ul>

Browser Check

Check browser version, JavaScript, Cookie, Adobe Flash Player version. Warn user if old.

sdesalas

This plugin allows WordPress to block over 95% of spam comments using a time-based hash.
This allows for a minimum sanity check and yet should remove almost all spam comments 
without the need to sign up to any third party APIs.
The plugin has been tested in real world conditions for around 12 months. It will not block 
real comments and it is pretty effective against spam, however it only uses a very simple 
algorithm and for that reason some spam comments may still make it through.

WP Simple SpamCheck

This plugin allows WordPress to block over 95% of spam comments using a time-based hash.

Browser Check vs WP Simple SpamCheck