Code:
<form name="form_cat" method="get" action="home.php">
<select name="cat" onChange="document.form_cat.submit()">
<option value="0">Select...</option>
{section name=cat_num loop=$categories}
<option class=CategoriesList value="{$categories[cat_num].categoryid}" {if $cat eq $categories[cat_num].categoryid}selected{/if}>{$categories[cat_num].category_name|escape}</option>
{/section}
</select>
</form>