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
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Extensions currently have very Greasemonkey-like functionality: you identify which web pages should be modified, plus JavaScript to be added to those pages. By default, the extension's JavaScript runs after the page loads, but you can specify that the extension's JavaScript should run before the page loads. Right now, you can only load one JS file, but that could change in the future. You also can't currently load Cascading Style Sheets (CSS), but that might also change.
The Chrome extension manifest, which has metadata about your extension such as name, version, etc., looks much simpler to me than how Firefox wants extensions to be packaged. That's a big plus in my book, because you spend most of your time writing code and not worrying about packaging up your plug-in. On the down side, I didn’t see any support for internationalization, which is one of the benefits of Firefox's more comprehensive way of packaging up plugins. Another limitation of the current Chrome extension spec is that you can’t do much other than modify pages via JavaScript. And I didn’t see a way to introduce new widgets into the actual “chrome” of the Chrome browser.
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From: Matt Cutts, mattcutts.com
Google Chrome extension tutorial for developers