Even though there are a few online communities about Chrome extensions a lot of people are probably missing out on extensions, first of all because they're not enabled at all in the stable channel and second because you previously needed a special flag in the shortcut even on the dev channel. If you haven't already I strongly urge any Chrome users to switch to the dev channel, it is much further along the development and though it says it can be very unstable at times is has been extremely rare for me. It can be done here: switch between stable/beta/dev channel. The flag in the shortcut isn't a problem once you have the dev channel anymore, last week when they finally made it the default to have extensions enabled in the dev channel (along with some UI changes I've been looking forward to).
Now, how do you use extensions in Chrome? It's really very simple. All you have to do is find a link to an extension and click on it (or if you have an extension file (.crx), you can just drag it to your Chrome shortcut). After accepting the download Chrome will pop up a message asking you if you want to install the extension, which of course you do (unless you distrust the source of course). After that the extension is installed instantly (no need to restart Chrome like you do with Firefox) and you should see any UI the extension adds. Regarding UI, there are four types of them:
- The toolstrip is a bar at the bottom of your browser window. It automatically allocates enough space to fit the extension since Chrome values the screen real estate of their users, something I greatly appreciate. In the toolstrip there can be basically anything, buttons, links, information, checkboxes, etc. The toolstrip can be hidden with the shortcut ctrl+alt+b. It's the most common form of extension UI.
- Moles are pretty much an extension of the toolstrip. When you hover over an extensions allocated space in the toolstrip the name comes up. When you click the name you can open a mole (if the extension has created one) which is kind of like a popup page that shown up above the toolstrip. You can, for example, use the toolstrip to display some information and a form with checkboxes etc. in the mole for setting up what and how to display things.
- Content scripts:
- Page actions:
- Page actions are used when you want the option to do things with a certain page. What it does is give you an icon in the omnibox (address bar) for the extension on the pages where it can be run. When you click on the icon the extension does something with the page. For example the extension can detect when a page has an RSS feed available and enable the page action for the page. An RSS icon will show up in the address bar which will e.g. add the feed to your Google Reader account when clicked.
An extension can combine any number of them in any combination and can communicate between them (as long as they belong to the same extension).
To test how extensions work I suggest you to visit the official sample extensions. As of now there are three sample extensions there:
Displays the number of unread mail in the toolstrip.
Subscribe in Feed Reader
Uses page actions to subscribe to a feed.
Shows the status of the Chromium Build Bot (probably not interesting to a regular user).
These are two simple extensions that I wrote for some basic SEO work:
Shows the PageRank of the open site in the toolstrip.
Shows the Alexa Rank of the open site in the toolstrip.
To find more Google Chrome extensions I suggest you visit Chrome Plugins. They've got discussions and a lot of extensions posted by users, for example for mouse gestures, translation, smooth scrolling, session saving, weather, alarm clock, video downloader, etc.
I hope this'll get anyone still using other browsers because they've got extensions they need to switch to Chrome - it's got a fair number of great extensions already, and an excellent and easy-to-use extensions API for creating new extensions in case there doesn't yet exist one with the functionality you require. :)