I use Pingdom and Nagios now that I "roll my own" software stack on linode.com's VPS service. Pingdom gives me a view from the outside world into my site. And Nagios, which is Open Source software installed on the web server, gives me both an internal view of problems within the web server, and an "inside looking out" view which, typically, tells me if there are network problems in the Data Centre where the web server is running. I've also set Nagios to monitor some things as quickly as every 10 seconds, which gives me a view of shorter duration outages than Pingdom's 1 minute checking will catch.