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
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.