[development] Module for scheduling work?

For a module I'm writing, I need a way for end users to be able to schedule tasks to happen (e.g. send out an email). Is there a helper module or PHP class out there that I can use to trigger these tasks being completed, both one time tasks and recurring tasks? Googling has found a few options but none were that satisfactory so I'm curious what other people have used for similar situations. Thanks -- Kyle Mathews Blog: kyle.mathews2000.com/blog Twitter: http://twitter.com/kylemathews Company: http://eduglu.com