Marco Beierer

<p>The <a href="https://www.marcobeierer.com/tools/link-checker/pro" rel="nofollow ugc">Link Checker</a> for WordPress uses an external service to crawl your website and find broken links and images on your website.</p>
<p>In contrast to search engine tools like the Google Search Console, which only show if a URL on your website is not reachable, it does not matter for the Link Checker if the links leads to an internal or external URL. The Link Checker will find all dead links.</p>
<p>The Link Checker works for every plugin out of the box. The computation costs for your website is also very low because the crawler does the heavy work and just acts like a normal visitor, who visits all pages of you website once.</p>
<h4>Limitations of the free version</h4>
<p>The Link Checker can be used for free, but the free version is limited. You can check websites with up to 500 URLs. Advanced features like image or video checks, CSV export, scheduler support, and form login support aren’t available in the free version.</p>
<p>The service cannot be offered free of charge for larger websites because the crawling is done by servers operated by the plugin developer and renting and maintaining these servers isn’t cheap.</p>
<h4>Videos</h4>
<p>You can find videos about the Link Checker on the <a href="https://www.youtube.com/watch?v=8rUFDp09tjs&list=PL5VYcNma6nfxIrlbpz0f1avXT_JIrUZA1" rel="nofollow ugc">Link Checker for WordPress playlist on YouTube</a>.</p>
<span class="embed-youtube" style="text-align:center; display: block;"><iframe loading="lazy" class="youtube-player" width="750" height="422" src="https://www.youtube.com/embed/8rUFDp09tjs?version=3&rel=1&showsearch=0&showinfo=1&iv_load_policy=1&fs=1&hl=en-US&autohide=2&wmode=transparent&listType=playlist&list=PL5VYcNma6nfxIrlbpz0f1avXT_JIrUZA1" allowfullscreen="true" style="border:0;" sandbox="allow-scripts allow-same-origin allow-popups allow-presentation allow-popups-to-escape-sandbox"></iframe></span>
<h4>Features</h4>
<ul>
<li><strong>Simple setup</strong>.</li>
<li><strong>Works out of the box</strong> with all WordPress plugins.</li>
<li><strong>Low computation costs</strong> for your webserver.</li>
<li>Lists all broken links and redirects on your website.</li>
<li>Support for Polylang (probably also WPML) sites.</li>
</ul>
<h4>Technical Features</h4>
<ul>
<li>Respects your robots.txt file (also the crawl-delay directive).
<ul>
<li>You can use the user-agents MB-LinkChecker to control the crawler.</li>
</ul>
</li>
</ul>
<h4>Additional Features of Link Checker Pro</h4>
<ul>
<li>Check if embedded internal and external <strong>images</strong> are broken.</li>
<li>Check if embedded <strong>YouTube videos</strong> are broken.</li>
<li>Scheduler to <strong>automatically check a website</strong> once a day and get a summary report by email.</li>
<li>The result can be exported as CSV file.</li>
<li>Form login support to check protected pages like membership areas.</li>
<li>The result is saved on the Link Checker server so that it can be fetched multiple times without requiring a recheck. For example from different users or with different browsers.</li>
</ul>
<h4>Upcoming Features</h4>
<ul>
<li>Support for checking the availability of videos, CSS files and JS files.</li>
</ul>
<h4>Technical Requirements</h4>
<ul>
<li>Works with <strong>WordPress 4.7 or newer</strong> on <strong>PHP 5.6 or newer</strong>.</li>
</ul>
<h4>Use of an External Server</h4>
<p>The Link Checker uses an external server, operated by the developer of the plugin, to crawl your website and detect broken links. This means, that there is some communication between your website and the server. The only data that is communicated to the external server by your website is the URL of your website and the fact that you are using WordPress. The server than crawlers your website (as a normal visitor does) and answers with a list of the found broken links.</p>


Link Checker Pro

EDITED REVIEW: 2021.03.10 
The free plugin works, but stops before it can finish being useful. 500 links is simply not enough for a free version. It doesn't provide a free tier that's functional for most medium-sized websites. You end up having to buy it to actually use it. That makes this more of an advertisement than an actual plugin. I'm using a different plugin.

EDITED 2021.03.11
See my reply dated today.

	EDIT: The free plugin only checks 500 links. This is entirely useless, even for medium-sized sites.
	EDIT: Plugin registration consists of link allotments, with the minimum being 25K links checked in one go for ~50 Euros. It goes up drastically from there because Marco's server is the one doing the crawling.
	Most target websites are small, hosted on huge commercial properties optimized for WordPress. Additionally, crawling the local site may not be as optimal as scraping each post's content field for links and storing results locally.
	My server should be the one doing the work. If my server is doing the work, you can't expect me to pay by the link for a service my server is doing. Period.
	An example of a per unit subscription model that works is Mailchimp. Their servers are doing the work in a trusted secure environment so that my web host box doesn't become a shadow spambot. Offloading that service to mailchimp is not sketchy, and their terms are more than generous for small to mid-sized website admins, with a free tier that is actually commodious. That's not what's happening with this plugin.
	It checks every link on every page, even footer links that repeat across the entire site, so if you have a link like "theme created by...", that link counts on every page against your priced link count. Absurd.
	Worse, it counts a number of known valid links as broken.
	WordPress plugins in the .org library should offer a reasonable modicum of usefulness out of the gate. It should not seem hampered to the site admin. The pro version should offer additional capabilities that make the free plugin seem more complete. This plugin offers only a glimpse of what the paid plugin might do if you are willing to pay 45 euros minimum to check 25K links. You want to check more links? Buy more tokens.
	EDIT: Given the Author's (Marco Beierer) balanced response, I'll say this would better serve the WP community and Marco himself by:

 	scraping post content directly for links rather than crawling, 
	removing the tier caps -- including for the free version -- and then 
	Price the pro version between $50 and $100 for a single-tier paid version that also includes image and youtube link checking.
	Also for the pro version, add the ability to check all site menu links, sidebar/footer/widget links, post content for custom post-type links, custom field links, etc.

This would give the expected additional capability along with unrestricted link checking found in the free version.

A great plugin makes the free-plugin adopter feel like they're getting such a bargain they feel guilty for not supporting the plugin with either a donation or an upgrade to the pro version.

Bait and Switch - Pretty much forced to buy if you want to use.

An easy to use link checker for WordPress to detect broken links and images on your website.

boborabit

<p>The idea of this plugin is coming from my work.<br />
I’ve been assigned a lot tasks for cleaning broken links.<br />
Which is so bored.<br />
Editors are lack of basic HTML skills, they always tried to insert links like such:</p>
<ul>
<li>href=”www.abc.com</li>
<li>href=”awesome-slug</li>
</ul>
<p>Apprently, they will finally result in</p>
<ul>
<li>http://mydomain.com/current-page/www.abc.com</li>
<li>http://mydomain.com/current-page/awesome-slug</li>
</ul>
<p>So this plugin is to display a warning message in WordPress dashboard,<br />
in case there is such invalid URLs in post content.</p>


Validate Link

A Very simple plugin for editors. It will outline any invalid links upon saving the post.

Link Checker Pro vs Validate Link