see Monitoring Applications in the Cloud article at SYS-CON Magazine:

Keeping track of all the pieces inside the cloud is very important, but you also need to know how your environment in the cloud is performing to the outside world.  There are more external monitoring services out there than I can count.  But I’ll tell you who we use.  Our favorite at the moment is monitor.us.  We like them because starting at just $10/month you get on demand fault & performance monitoring for your environment. This external watchdog system helps to keep everyone informed if/when the cloud is having issues.  It also provides us with important statistics on response time and application performance that we use to determine how to adjust our infrastructure.