category tree

@@hthis part goes into the template:

<?php
hierarchical_category_tree( 0 ); // the function call; 0 for all categories; or cat ID
?>

@@hthis goes into functions.php of your theme

function hierarchical_category_tree( $cat ) {
    static $depth=0;
    // wpse-41548 // v1.3 // structured nested list // alchymyth // a hierarchical list of all categories //
    $next = get_categories('hide_empty=0&orderby=name&order=ASC&parent=' . $cat);
    if( $next ) :
        $depth++;
            echo "\n";  for($i=1;$i<$depth;$i++) echo '   '; //for structured html output
        echo '<ul class="depth-'.$depth.'">';
        foreach( $next as $cat ) :
            echo "\n ";     for($i=1;$i<$depth;$i++) echo '   '; //for structured html output
        echo '<li><a href="' . get_category_link( $cat->term_id ) . '" title="' . sprintf( __( "View all posts in %s" ), $cat->name ) . '">' . $cat->name . '</a>';
        hierarchical_category_tree( $cat->term_id );
        echo '</li>';
        endforeach;
            echo "\n";  for($i=1;$i<$depth;$i++) echo '   '; //for structured html output
        echo '</ul><!--/.depth-'.$depth.'-->';
        $depth--;
            echo "\n ";     for($i=1;$i<$depth;$i++) echo '   '; //for structured html output
    endif;
}