I've been thinking about building something like Exceptioneer for a while. I'm always vaguely jealous of nice simple services for developers that do something useful. They are generally always things that you could do yourself but its just easier to have someone else work out all the bugs.
Pingdom is another one that's smart, especially their pricing model. You only pay for blocks of texts, the implication being that if your site is up all the time you won't have to pay.