Alexandru Vornicescu

If you want to exclude some post types or terms from the search page then this plugin is for you.

Filter Search Page

This plugin will help you to filter the search page results by category and post type.

Tom Auger

This plugin, once activated, spits out the information passed by each filter used in manipulating database queries, as a learning tool.
The code itself is heavily documented and demonstrates best practices for working with those filters and actions to view / manipulate the query.
It is very important to note that this plugin is not meant for a production environment as it will insert a lot of really ugly HTML throughout your page. You’ll get the most use out of this plugin by looking at its code, and maybe previewing one or two pages on a sandbox, vanilla WordPress site.

Valz Display Query Filters

This demonstration plugin identifies the primary filters that are used by WordPress as it parses your queries.

Filter Search Page vs Valz Display Query Filters