Check if wp-login is current page

Use the global $pagenow, which is a common global set by WordPress at runtime:

if ( $GLOBALS['pagenow'] === 'wp-login.php' ) {
    // We're on the login page!
}

You can also check the type of login page, for example registration:

if ( $GLOBALS['pagenow'] === 'wp-login.php' && ! empty( $_REQUEST['action'] ) && $_REQUEST['action'] === 'register' ) {
    // We're registering
}

Following code is considered legacy and should not be used (wp-register.php was deprecated & subsequently removed quite a while back):

if ( in_array( $GLOBALS['pagenow'], array( 'wp-login.php', 'wp-register.php' ) ) )
    run_my_funky_plugin();