<h2><?php _e('Posts by Category'); ?></h2>
<form action="<?php bloginfo('url'); ?>/" method="get">
<div>
<?php
$select = wp_dropdown_categories('show_option_none=Select category&show_count=1&orderby=name&echo=0');
$select = preg_replace("#<select([^>]*)>#", "<select$1 onchange='return this.form.submit()'>", $select);
echo $select;
?>
<noscript><div><input type="submit" value="View" /></div></noscript>
</div></form>