<script language="JavaScript">

function loadPage(list) {
  location.href=list.options[list.selectedIndex].value
}

</script>

<div id="sidebar">
    <h4>Categories</h4>
    <ul>
    <?php wp_list_categories('show_count=1&title_li=&hierarchical=0'); ?> 
    </ul>   	    	
<div id="sidebar-footer"></div>
</div>

<div id="sidebar">
	<h4>Monthly Archives</h4>
	<form>
	<select onchange="loadPage(this.form.elements[0])">
	<option selected="selected">Select --</option>
	<?php wp_get_archives('type=monthly&format=option'); ?>
	</select>
	</form>
<div id="sidebar-footer"></div>
</div>