Tracy Rotton

By default, the “Search” widget in WordPress will include the searchform.php template file in your theme (if it exists), or the search code in wp-includes/general-template.php (if it doesn’t). However, if you want a distinctive search form (or forms!) to use as a widget, this allows you to enter it in using the “searchform-identifier.php” format that you can use to distinguish other templates elsewhere in your theme. You just have to have that file set up in your theme with the HTML you want to use.
Example: If you want to have a distinct searchform file specifically for your widgets, create the template file and call it something like “searchform-widget.php” (what goes between the dash and the dot is up to you, so long as it meets filename requirements). In the “Template” field of the widget settings, enter only “widget”, and that searchform file will be called in your widget automagically.
Note: Actually coding the search form is up to you. That requires some theming skillz, but I have confidence in you!
To Do: Make a short-code version of this, for use in pages and posts.

TC Specify Search Form

Replaces the default WordPress "Search" widget with one that will use a customized searchform template in your theme.

Arefly

Sometimes when user are using the search function of your blog. They may not know the full string of they want to search.
And this plugin will add a Search Keyword Hint (get hints from all tags) on the search form of your site.
So that user can find the things they want to search more easily.
Live Demo: <a href="http://www.arefly.com/" rel="nofollow ugc">Click Here</a> (Try enter any word in the search box.)
中文介紹請看<a href="http://www.arefly.com/wp-search-auto-match/" rel="nofollow ugc">這裏</a>

WP Search Auto Match

Add Search Keywork Hint like Google Search to your blog's search form.

TC Specify Search Form vs WP Search Auto Match