Custom filter on all post page (admin backend)

Use the restrict_manage_posts action to add other HTML content beside the Category/Date dropdown filters. Try something simple like:

add_action( 'restrict_manage_posts', 'my_posts_restrictions' );
function my_posts_restrictions() {
    echo 'xxxxx';
}

This should give you xxxxx at the end of yoru filters line. Be warned, as this will appear on any Custom Post Types as well. You need to check your current_screen to make sure you are on the Posts list table, and not any others (like Pages for example).