(1) An Internet service that pings computers on a regular basis to determine if they are still running.  For example, a ping service would be used with a server farm to determine if any of the servers have failed.  See 
ping.
(2) A service on the Internet that receives notifications (pings) from blogs that a new post is online.  All subscribers signed up for that blog are notified of the update.  Ping servers generally provide the service at no charge.