How to Quickly Tell if a Site Is Down for Everyone or Just You

On many occasions, the frequent surfers among us will have struggled to view a website that refuses to load. This raises a simple but profound question: “Is this website down for everyone or just for me?”
Now, you could call someone or send out an email asking if others are having the same experience. But, you might get a quicker response with a tweet. But even better yet, there is a humble website that will answer this important question: www.downforeveryoneorjustme.com.
What will they think of next?