Given the amount of people that message me on IM to check if Paypal, their blog or another website is also down for me, I suspect that not every one know this little handy tool. (Not pointing fingers, I used to harass people all the time when my websites went down as well…).
Downforeveryoneorjustme.com will basically perform a quick check to see if the website you are currently seeing down is actually down for everyone else or just for you. This is very useful because sometimes your website will appear to be down, but in reality you might be having ISP or DNS problems, and you are the only person not being able to see it. In those cases, obviously, this tool could help you not to freak out.
I am not really sure how accurate the tool is (this depends on the number of servers they are using, and on their physical distance and IP diversity), but from basic tests I performed it will report the real situation most of the times.
There you go, bookmark it, and the next time you see a website down, just paste the URL there to see what is going on.