Changing the category permalink structure

The following code changes all links to category archives so they don’t include the parent category:

add_filter( 'category_link', 'wpse7807_category_link', 10, 2 );
function wpse7807_category_link( $catlink, $category_id )
{
    global $wp_rewrite;
    $catlink = $wp_rewrite->get_category_permastruct();

    if ( empty( $catlink ) ) {
        $catlink = home_url('?cat=" . $category_id);
    } else {
        $category = &get_category( $category_id );
        $category_nicename = $category->slug;

        $catlink = str_replace( "%category%', $category_nicename, $catlink );
        $catlink = home_url( user_trailingslashit( $catlink, 'category' ) );
    }
    return $catlink;
}

Add the code to the functions.php of your theme.