http://www.PushMon.com alerts you if something doesn’t happen. Specifically, it alerts you if your custom URL isn’t hit on user defined conditions.
For example, you can create a custom URL and tell PushMon to alert you if it has not been access by 5:00AM CST daily. In your server jobs, you simply send a request to the URL (using cURL or wGet) if no errors occured in your jobs.
I’ve been using the service for about 6 months and it works like a charm.
As you can see in the following picture, I have specified a "deadline" for my custom URL.
This is the setup screen for a custom URL. PushMon has numerous alerting capabilities.
If you only want alerts when something goes wrong, check out PushMon.com. I recommend it.