Posted by Andrew.
The Chromium extension system is in active development. Google published a Chrome extension HOWTO tutorial for developers. However, it is not yet ready for use by the general public. The majority of the APIs are not implemented yet, and there is no UI, no autoupdate system, etc.
There is a Getting Started tutorial for Chromium developers, it shows how to create a simple extension that shows Hello, World! in Chrome. First of all, you will need to be using either the developer channel of Chromium or else a recent trunk build.
Matt Cutts also posted an article How to Write a Chrome Extension in Three Easy Steps in his blog:
How to Write a Chrome Extension in Three Easy Steps
From: Matt Cutts, mattcutts.com