Safari 5 was a significant version upgrade for pretty much one reason: Extensions. The other improvements to Safari were performance-related; nice, but not earth-shattering. The Extensions support has finally brought sorely lacking functionality to the Safari web browser as compared to other browsers like Google Chrome and Firefox.
The official Apple resource for Extensions is the Safari Extensions Gallery, which features all of the Apple-approved Extensions for Safari. Another great resource, which showcases many Extensions not on the Safari Extensions Gallery, is the Safari Extensions blog.
Extensions are simple to install. In the Safari Preferences, go to the Extensions pane. From here, you can enable (or disable) Extensions support entirely; you can view the Extensions you have installed and uninstall them if you prefer; or tweak configurable options for those that are installed, and also quickly see if any updates for your currently-installed Extensions are available.
You can elect to have Safari automatically install new updates as they are available, or you can check every once in a while yourself to see if any are available and update them. Unlike Firefox, Safari does not tell you upon startup if new Extensions are available to update -- this is a pretty big oversight on Apple's part as it leaves you to remember to check the preference pane every once in a while, or have it handled automatically. The other oversight is that Safari doesn't tell you when it does update them automatically: no warning, no note, no heads up, and no log to see what was updated and when.
The mechanism of updating and installing Extensions is pretty simple, so we've got some room to highlight a few useful extensions.
Autocomplete is an enhancement, or override might be a better name, to the standard Safari AutoFill feature. Some sites prevent you from auto-filling certain web forms (financial sites and others), but if you are in a private location such as at home, and are willing to take the risk for convenience, this Extension will allow you to force autocompletion, even when the site tries to restrict it.
For web developers, the BetterSource Extension gives you much more information on viewing a page's HTML source than the standard Safari "view source" view does. It opens the source in a new tab, has syntax highlighting, shows line numbers, and can also show you the original source vs. generated source (what it looks like after Safari and/or any Extensions have altered it for rendering).
Sessions is a pretty slick Extension in that it lets you manage, remember, recall, and manipulate current and previous sessions. You can easily restore sessions when Safari starts, or you can open a previously-saved session from last week or last month.
Finally, Safari Access Keys is an Extension to let you easily see what keyboard shortcuts are available on a site. The shortcuts are displayed on a toolbar for easy access and can be executed by pressing CTRL-OPT-[KEY].
There are a number of other Extensions for Safari and most of them have to do with customizing the display of specific web pages. Some are useful, some not so much, so a little playing around with various Extensions will help you choose the ones that enhance your productivity or improve sites you visit.
Compared to Chrome or Firefox, Safari has a really small number of Extensions, but that number is quickly growing.
Vincent Danen works on the Red Hat Security Response Team and lives in Canada. He has been writing about and developing on Linux for over 10 years and is a veteran Mac user.