Check any Internet host's reachability and how much latency there is in communicating with it from 50 locations worldwide

When visiting a a web site produces a 404 or another message, check if it's that site that isn't working (down for everyone) or if it's your Internet service provider (just you).

Ping is commonly used to determine if a given host is 'alive' on the Internet. It will repeatedly send a datagram to a network host to elicit a response from it and show you if, and how long it took to respond.