Emails the site administrator whenever a user logs in.

track-login-ips.php

/**
 * Track User IP
 *
 * Emails the site administrator with the logged in user's IP
 * address. Triggers whenever a person logs in.
 *
 * @param string  $user_login Username of logged in user
 * @param WP_User $user       User object
 *
 * @return void
 */
function ng_track_user_ip( $user_login, $user ) {
	$ip          = ng_get_ip();
	$admin_email = get_option( 'admin_email' );
	$message     = sprintf( __( '%1$s logged in from IP address: %2$s' ), $user_login, $ip );

	wp_mail( $admin_email, __( 'New Site Login' ), $message );
}

add_action( 'wp_login', 'ng_track_user_ip', 10, 2 );


/**
 * Get IP Address
 *
 * Gets the IP address of the current user.
 *
 * @return string
 */
function ng_get_ip() {
	if ( ! empty( $_SERVER['HTTP_CLIENT_IP'] ) ) {
		$ip = $_SERVER['HTTP_CLIENT_IP'];
	} elseif ( ! empty( $_SERVER['HTTP_X_FORWARDED_FOR'] ) ) {
		$ip = $_SERVER['HTTP_X_FORWARDED_FOR'];
	} else {
		$ip = $_SERVER['REMOTE_ADDR'];
	}

	return $ip;
}