I put this together to use for a couple of personal projects and thought I'd see if there's any interest in something like this as a pay service.
It's essentially a hosted REST-like API for job scheduling - give it a cron expression or a single time in the future and it notifies the application by HTTP/S callback at the appropriate moment(s). You can provide an arbitrary payload to be delivered along with the callback containing identifying state.
I'm using it mostly for scheduling test executions and user lifecycle management, but I imagine it could be useful for quite a bit more.
So, is there any demand for this? If so, perhaps I'll take part in the launch-in-November push going on now.
I threw together a document on the API with more details - comments welcome!
http://job-scheduler.info/