Here are a few tools that you can use to find out if a site is down. Some of these tools will also notify you when it goes back up again.
Ding It’s Up: Get email, SMS and Twitter notification when a site goes down and when it’s up again.
NotifyMee: It sends you an email when a downed site you want to visit reappears on the internet. It follows a downed site and lets you know when it returns. It should be very useful next time your favorite weblog is unreachable or a link you really want to check out crashes under the weight of its popularity. It also outputs a tweet to notify other people that the site is down and when it goes back up.
IsitFucked: A simple tool that you can use to find out if a website is down.
Is this Down: Another tool to check if a site is down for everyone or if it’s only down on your computer or your location.
DownforEveryoneOrJustMe : Same as Is This Down but without the ads.
Down For You Too: Another website status checker. It also displays a list of recent checks. If the domain that you are going to check happens to be on the list and it’s down, there’s no need for you to know if it’s just you.