Code:
<form method="get" action="home.php">
<select name="cat" onchange="this.form.submit()">
<option value="0"{if $cat eq "0"} selected{/if}>{$lng.lbl_categories}...</option>
{foreach from=$categories item=c}
<option value="{$c.categoryid}"{if $cat eq $c.categoryid} selected{/if}>{$c.category}</option>
{/foreach}
</select>
</form>