George Pattichis

Link Juice Keeper redirects all non-existing URLs, that normally return a 404 error, to the page/post of your choice. This way everyone who comes to your blog via a broken link (both people and robots) will be redirected to a valid page.
A 404 Error (Page not Found) is very common and one of the main reasons Google can give you a lower ranking. Taking steps to handle 404 errors can be complex for non-techy users. With this free plugin you can finally resolve all 404 errors with just 1 click!
<h3>What are 404 errors?</h3>
As you probably know, incoming links play important role in ranking well in Google and other search engines. Therefore you should ensure that every incoming link leads to one of your valid pages on your blog. This may be a challenge because your website is dynamic and it changes every day:
<ul>
<li>you might remove or unpublish posts or pages;</li>
<li>you might change your URL structure on your blog;</li>
<li>links may brake because of technical misconfiguration;</li>
<li>a webmaster might add your link in their posts with a typo;</li>
<li>a visitor may simply mis-type your post’s URL</li>
</ul>
Any of the above will lead to link juice waste and a 404 error. You can avoid this simply by installing this plugin.
<h3>Features</h3>
<ul>
<li>Redirect errors to your homepage, an existing page, post or even a custom link</li>
<li>Monitor/log all 404 errors (Optional)</li>
<li>Choose redirect method to be used (301,302,307)</li>
<li>Send email notifications on every 404 (optional)</li>
<li>Translations ready</li>
</ul>
Link Juice Keeper was originally developed by <a href="https://profiles.wordpress.org/sirzooro/" rel="nofollow ugc">Daniel Frużyński</a>. The plugin has been adopted and refactored by <a href="https://profiles.wordpress.org/pattihis/" rel="nofollow ugc">George Pattichis</a> who will continue development and maintenance.
<a href="https://wordpress.org/extend/plugins/link-juice-keeper/changelog/" rel="ugc">Changelog</a>

Link Juice Keeper

Improve your SEO and keep your link juice by automatically redirecting all 404 errors to any page/post/url. User friendly options and log feature.

phpgeekfr

This plugin logs 404 (Page Not Found) errors on your WordPress site. It also logs useful informations like referrer, user address, and error hit count. It is fully compatible with a multisite configuration.

404 Error Monitor

The plugin causes taxes to fail on Woocommerce, so incorrect rates can be returned.

It throws this error:

Deprecated: Methods with the same name as their class will not be constructors in a future version of PHP; errorMonitor has a deprecated constructor in /www/wp-content/plugins/404-error-monitor/index.php on line 51

Otherwise, it's a great plugin and does what it says, but it needs an update to fix this issue with Woocommerce.

Good for what it does, but causes a tax issue with Woocommerce

This plugin logs 404 (Page Not Found) errors on your WordPress site.

Link Juice Keeper vs 404 Error Monitor