Some developer resources that I find useful.

codex.wordpress.org 
generatewp.com
wp-hasty.com

Lukes awesome WP developer tools list:
github.com/lukecav/awesome-wp-developer-tools

Additional resources:
asphaltthemes.com/wordpress-developer-resources

I will add on to the list when I came across additional resources.

When I need to know the host of a specific web site.
www.whoishostingthis.com/
hostadvice.com/tools/whois/

Boiler Plates:
designyourway.net/blog/wp/wordpress-boilerplates

smashingmagazine.com/2015/06/wordpress-custom-page-templates

Tools that look useful:
localsync.io

One language in the backend and one in the frontend:
gist.github.com/scarstens/11378999

   // setup one language for admin and the other for theme
     // must be called before load_theme_textdomain()

     function set_my_locale($locale) {
          $locale = ( is_admin() ) ? "en_US" : "it_IT";
          setlocale(LC_ALL, $local );
          return $locale;
     }
     add_filter( 'locale', 'set_my_locale' );

www.philowen.co/blog/add-new-admin-user-to-wordpress-via-phpmyadmin/

Create a simple 404.php file to redirect to front page.

Create a 404.php file containing:

<?php
header("HTTP/1.1 301 Moved Permanently");
header("Location: ".get_bloginfo('url'));
exit();
?>

Place the file into your child theme root folder. Test it out by going to the domain name and /thensomething. If it is not an actual page or post it will redirect to the front page.  www.wpbeginner.com/wp-tutorials/how-to-redirect-your-404-page-to-the-home-page-in-wordpress

Check my other article Useful site helper tools.