I've been using RevealCloud for about four months and it's pretty sweet. I've been through a few server monitoring tools over the last 10 years and all of them tell me there's a problem with the site when it's too late to actually fix it with SSH. Mostly it's because I've let the disk fill up or there's a memory leak in Apache (or more often silly old lighttpd), so it's good to know about stuff starting to break. Which is just what RevealCloud does. It runs a little process and monitors stuff, sends it to their server (so you get nice graphs of that stuff) and sends you an email when certain things are outside boundaries you set up. It works extremely well. And so far I've had no false alarms, which is so terribly wonderful.
I've started turning off my other monitoring software (especially Monitis, which I was actually paying for).