Site translators are the secret sauce that make Zotero so easy to use. Here you'll find the documentation to get you started creating your own site translators.
How to Write a Zotero Translator
A fairly comprehensive tutorial for beginners, written by Adam Crymble, describing how to write a translator using Scaffold. Limited to screen scrapers.
Scaffold: an IDE for Zotero translators (only compatible with Zotero 1.0.x)
Scaffold is a Firefox extension designed to help you write translators.
Zotero Translators - The Missing Manual This missing manual aims to expand on Adam Crymble's introductory tutorial, discussing the functions in Zotero.Utilities, describing how translators can call other translators, and more.
A more technical (but fairly outdated) guide additionally covering the creation of import and export translators can be found here