I've wanted this for years, but apparently never enough to build it myself:
A passive monitoring tool written in Python & Django. Set up your cron jobs, backup scripts, weekly email sending scripts, nightly data import jobs etc. to ping this service when they complete. When they don't send a ping on time, you receive an alert.
The service offers a generous 20 free checks before you start paying. And since it's an open source Django app, you can set it up to run on your own infrastructure too.