sixthStation

Add a little search box to category lists
<h3>Remove plugin</h3>
<ol>
<li>Deactivate plugin through the ‘Plugins’ menu in WordPress</li>
<li>Delete plugin through the ‘Plugins’ menu in WordPress</li>
</ol>

Sixth Station Category Search Box

Add a little search box to filter category lists

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.

Sixth Station Category Search Box vs WP Meta Search