Periodic is a fully unit tested PHP based task runner. It is supposed to deliver a basic implementation for managing all kinds of recurring tasks and events inside your web application. It has been designed with having all kinds of different web hosting environments in mind. It is capable of running on most shared hosting systems as well as root servers.
They even went so far as to write up a design document for the implementation. You can grab the latest version of the script off of their subversion server: svn://arbitracker.org/arbit/projects/periodic