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.

Andrew Norcross

tripflex

Creates a panel to change or delete meta keys in bulk. Useful for when you are switching plugins or themes that use specific meta keys for functionality, or for general cleanup for older sites that may have older meta data that is no longer in use.

Post Meta Manager

A simple utility plugin for changing or deleting post or user meta (custom fields) keys in bulk.

WP Meta Search vs Post Meta Manager