Y.Kohno

This is a flexible search form widget. 
You can easily generate a multi condition search form and display anywhere as widget.
複数のカスタムフィールド、投稿タイプ、カテゴリー、タグでの検索が可能な複合検索フォームを簡単作成できるプラグイン。ウィジェットで好きな場所に設置可能。
Currently supports 
– Custom Fields 
– Taxonomies 
– Categories 
– Post tags 
– Post Type

WP Meta Search

Search form for searching through custom fields, post types, categories, tags, and free text.

This replaces your standard custom post meta form. Allows extension of page, post and custom post type by allowing you to create customised post form fields when adding or editing post types.
This is typically for developers to extend your typical post and post type form. You can use get_post_meta function in your theme template to display the saved data.

WP Extra Fields

Allows extension of page, post and custom post type by allowing you to create customised form fields to include when adding or editing post types.

WP Meta Search vs WP Extra Fields