Downtime means loss of traffic, which can mean losing your audience or prospective buyers (if your site is a shop) which means if your site goes down you want to know about it. There are a number of online services that provide such information for you.
One new contender is Are My Sites Up? which is a nicely designed site and easy to use - however it seems a bit useless at the minute. I've registered four sites with them and chose to have updates to notify me when the site goes down and when service is restored by email and SMS. Today two of my sites went down, but didn't get an SMS or an email. When they came back up I got two emails for both sites (yes, four emails) to say they were back up. I wouldn't say that's much use really but I'm guessing as they're relatively new they're still having issues with emailing and SMS. Definitely one to keep an eye on as once they've got the issues worked out (assuming there are other people experiencing the issues too) it'll be a good tool. Until then, I've disabled my alerts from here.
So, onto the next service - mon.itor.us. This one has been around for over a year now and it seems pretty reliable. Their site is a little busier in terms of it's design, but it does actually work. They also have a lot more features as it's not just testing to see if the site is up, but also provides detailed reporting on uptime and advanced connection settings so you can specify different ports to test, etc. You can also setup multiple emergency contacts in the case that your site goes down. For these you have the option of providing and email address, IM account, and/or a mobile phone number for SMS.
The third option is for more technically minded folk, as it's an Apache module called Nagios. This one is intended more for Enterprise clients - it isn't pretty and it's something you have to install yourself on the server so isn't something you can get installed on shared hosting packages, etc. it has to be a dedicated server or one you root (super user) privileges to. Another advantage of Nagios is that it has a developer community who work on making extensions for it - very useful if you want to customise it to suit your needs!
