wp_login_form(); // Codex: http://codex.wordpress.org/Function_Reference/wp_login_form
// or:
wp_loginout(); // Codex: http://codex.wordpress.org/Function_Reference/wp_loginout
if ( ! is_user_logged_in() ) // Codex: http://codex.wordpress.org/Function_Reference/is_user_logged_in
wp_register('', ''); // Codex: http://codex.wordpress.org/Function_Reference/wp_register