Possible Duplicate:
Can anyone recommend a website monitoring service?
What do you use for website/service monitoring, and notification from a 3rd party?
Possible Duplicate:
Can anyone recommend a website monitoring service?
What do you use for website/service monitoring, and notification from a 3rd party?
If you have a more complex website (which I define as a site with more "stuff" that can break, such as Javascript, AJAX, Flash,...) , consider adding real user monitoring (RUM) to the mix. That is transaction monitoring using real web browsers like IE and Firefox.
We use AlertFox. They also have a useful free plan to get started: http://www.alertfox.com/free-website-monitoring/
Other good options for RUM are Keynote or Gomez, but they are "high end" (read: expensive).
I like Uptime Robot. Lots of notification options (SMS, e-mail, Twitter ...), up to 50 sites/services, and free.