MrXHellboy

Track your 404 hits!
Do you keep track of your 404 not found hits ? If you don’t, try this simple plugin. 
It will keep track of the hits on your 404 not found page.
Useful for what ?
Well, probably you removed or relocated some categories, pages, posts etc etc a while ago or maybe just a couple of weeks without knowing anymore. 
Right now it sucks search engines already checked your website and took a notice of those URI’s or maybe you have some anchors on your own website pointing to moved URI’s. 
Those visitors arrives on your 404 not found page with a notice that the page they are looking for not exists anymore. 
Some visitors will stay on your site and look for the particular page, but the other group leaves immediately. What a shame…
What does this plugin do ?
This plugin tracks those visits on your 404 – not found page. It stores the time, requested URI and Referer. 
More information is absolutely not needed. The time and referer does already overdo it, but maybe you can get some extra information out of it. 
Thos information is stored in a txt file since i found it quite useles to create a new table or use the database in the first place. 
With 1 single click on the button you empty the file when you’re done with taking the appropriate actions on the results. 
You may want to add rewrite conditions @ your .htaccess file or use a seperate plugin for adding redirects. 
You can check the report within the admin settings menu – 404 report
How do i get it to work ?
Simple! 
1) Upload this plugin into your plugin directory. Activate the plugin: admin->plugins. 
2) Open your 404.php page within your theme and add the following snippet somewhere on that page. 
[php]
404 reporting
if (class_exists(‘NotFound_404’)){ 
 $Error404 = new NotFound_404(); 
 $Error404->Log404Error(); 
} 
[/php] 
3) Change the permissions (CHMOD) of the file 404.txt to 600.
Have fun logging!
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404 Not found report

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.

404 Not found report vs 404 Error Monitor