The almighty DD32, who is responsible for a number of WordPress features including the plugin updater and installer, has put up a quick documentation of the WordPress Plugin information API (used to fetch informations from the official plugin repository). WordPress hackers: bookmark it. (And, ho, by the way: happy new year everybody:)
Overview of WordPress’ Transients API
There's a cool (and underused by plugins) API in WordPress: the Transients API. Transients are temporary options, ie options set with an expiration time. Anytime you're storing options and they have a short life span, you should use transients instead.