MrXHellboy

This plugin offers your visitors the possibility search on post tags. 
All you have to do is install this plugin and create a page and add the following shortcode: [tagsarchive]
The following CSS classes and divisions are added to this plugin so you are able to customize the look in your own stylesheet. 
This because when i update the plugin, no work will be lost on your side.
form#form-search-tags, span.error, form#form-search-tags legend, form#form-search-tags fieldset, span.headings, form#form-search-tags h3
<h3>Development Blog</h3>
<a href="http://online-source.net/category/scripts/wordpress-plugins/" title="Online-Source WordPress plugins" rel="nofollow ugc">Online-Source WordPress plugins</a>
<h3>Development</h3>
Version 1.1: Fixed: error message added if both fields (tagname like and tagname is) are empty. 
Version 1.2: Fixed: shows no tags anymore which have a usage of 0.

Advanced tag search

This plugin offers your visitors the possibility search on post tags.

Mathieu Chartier

En français
WP-Advanced-Search fonctionne en anglais et en français 
WP-Advanced-Search fonctionne avec un code court <?php WP_Advanced_Search(); ?> ou le shortcode [advanced-search] à ajouter dans la page de recherche du thème WordPress (search.php). Tout est entièrement paramétrable dans les réglages de WordPress (sous-menu ‘Advanced Search’). Disponible aussi pour WordPress MU (multisites).

Quelques exemples d’options existantes :
<ul>
<li>Choix des colonnes de recherche</li>
<li>Ordre des résultats modulable</li>
<li>Utilisation d’un algorithme de pertinence pour le classement final</li>
<li>Plusieurs types de pagination (classique, par clic avec “trigger” ou scroll infini)</li>
<li>Mise en surbrillance ou non des mots recherchés</li>
<li>Correction automatique des recherches</li>
<li>Autocomplétion et ajout automatique de mots dans l’index</li>
<li>Gestion des blocs à afficher dans les résultats</li>
<li>Plusieurs thèmes disponibles</li>
<li>Possibilité de rechercher une expression précise (mots entre guillemets)</li>
<li>Possibilité de rechercher des articles placés dans des catégories précises</li>
<li>…</li>
</ul>
Failles et vulnérabilités analysées par Florian Hauser de <a href="https://www.code-white.com/en/" rel="nofollow ugc">www.code-white.com</a>, merci à lui !
For English people
WP-Advanced-Search works in English and in French 
WP-Advanced-Search works with a simple code <?php WP_Advanced_Search (); ?> to add to the search page of WordPress (search.php). Everything is fully configurable in the settings of WordPress (sub-menu ‘Advanced Search’). Now available for WordPress MU !

Some examples of existing options:
<ul>
<li>Choice search columns</li>
<li>Flexible order results</li>
<li>Using a relevance algorithm for the final classification</li>
<li>Several system to display results (pagination, trigger or infinite scroll)</li>
<li>Highlight search terms, or not</li>
<li>Automatic correction of searches</li>
<li>Autocomplete option and automatic addition of words in the index</li>
<li>Manage blocks to display in the SERP</li>
<li>Several templates for result pages</li>
<li>Ability to write an exact search with words in quotation marks</li>
<li>Opportunity to show the results of specific categories</li>
<li>…</li>
</ul>
Vulnerabilities analyzed by Florian Hauser from <a href="https://www.code-white.com/en/" rel="nofollow ugc">www.code-white.com</a>, thanks to him!

WordPress WP-Advanced-Search

Moteur de recherche complet et puissant entièrement modulable (complete and powerful search engine, fully scalable).

Advanced tag search vs WordPress WP-Advanced-Search