WP-Cron is a plugin to support periodic execution of actions / plugins / stuff! From the readme:
What with human beings, search engine spiders, and content aggregators, we can be fairly sure that even the most uninteresting (public) blog will be visited with some regularity. WP-Cron relies on this regularity to schedule the execution of three new plugin hooks. These hooks execute roughly once every fifteen minutes, every hour, or every day.
Included in the download are three example plugins to demonstrate how to use WP-Cron:
Download WP-Cron 1.1a! Note: I dropped the revision on this version back to 1.1a, in order to make it easier for me to manage version numbers at the WordPress Plugin repository.
Follow developments of WP-Cron here!