Here is an example how you can add search box in sidebar.
To be used at your own risk.

    * Create a backup of all refered files
    * Create a new file and name it search.tpl in /template/<yourtemplate>/sidebar_modules/
    * Copy the following code into search.tpl

=========================search.tpl==============
<div class="featurebox">
	<div class="tlb">
		{php}
			echo "<span><a onclick=\"new Effect.toggle('s7','blind', {queue: 'end'}); \"> <img src=\"".my_pligg_base."/templates/".The_Template."/images/expand.png\" alt=\"expand\" /></a></span>";
		{/php}
        <a href="{$URL_login}">Search{#PLIGG_Visual_Search_Title#}</a>
        </div>
        <div id="s7">
	<form action="{$my_base_url}{$my_pligg_base}/search.php" method="post" >
		{if $templatelite.get.search neq ""}
		{assign var=searchboxtext value=$templatelite.get.search|sanitize:2}
		{else}
		{assign var=searchboxtext value=#PLIGG_Visual_Search_SearchDefaultText#}			
		{/if}
		<input type="text" size="8" class="SearchField" name="search" id="searchsite" value="{$searchboxtext}" onfocus="if(this.value == '{$searchboxtext}') {ldelim}this.value = '';{rdelim}" onblur="if (this.value == '') {ldelim}this.value = '{$searchboxtext}';{rdelim}"/>
		<input type="submit"  value="" name="" alt="" class="go" title="" />
		</form>
	</div>
</div>
==========================search.tpl=============

    * Save the file.
    * Open and edit sidebar.tpl file found in /template/<yourtemplate>/
    * Append the following code where ever you want the search box to appear in sidebar.

==========================sidebar.tpl============
Anywhere after:
{if $user_authenticated ne true} {assign var=sidebar_module value="login"}{include file=$the_template_sidebar_modules."/wrapper2.tpl"} {/if}

Add:
{assign var=sidebar_module value="search"}{include file=$the_template_sidebar_modules."/wrapper.tpl"}

==========================sidebar.tpl============

    * Thats it. Visit your website to see the search box right in your sidebox.