The most useful Growl Extras I personally recommend installing are just two out of five things from the Extra's folder: GrowlMail and GrowlSafari. But feel free to install more extras if you wish.

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GrowlMail Apple's Mail app is a great desktop client to handle your emails. But I often times miss the bouncing icons and badge numbers in the dock because they're simply too short an animation and too small a badge for me to notice. As a result, I often times overlook important emails that need prompt replies while I surf the web, or busy with another app.
With GrowlMail, Growl will be able to handle events for received emails and even RSS feeds.

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To install GrowlMail, follow the GrowlMail Installation.rtf file or just double click
GrowlMail.mpkg to start the installation wizard. Once done, a
GrowlMail entry should appear in the Applications tab in the Growl preference pane 
. We will configure this entry later on in this article to show you how you can customize Growl further by specifying additional sound events on top of Growl's display notifications.
GrowlSafari
Next, we will be installing GrowlSafari.
Safari, Apple's internet browser app, is another great app built into Mac OS X that can handle all your surfing needs. And it's a common occasion for every internet user to download content from the web - PDFs, installers, photos, office documents and the like. Safari provides its users a dedicated download manager window so users can have a centralized location to monitor downloaded media.
All of these downloads go straight to Mac OS X's download folder. Once a data transfer from the net to your computer is completed, the file is saved in the Downloads folder. Although Mac OS X has its own way of notifying the user that a download is completed where the Download folder bounces in the dock, I still tend to miss this animation since I most of the time continue surfing the web while waiting for a download to complete.
With GrowlSafari, Growl will be able to notify me unobtrusively that a download has completed when using Safari. The GrowlSafari extra adds Safari to be part of the Growl notification system.
The installation is pretty much the same with GrowlMail. Just double click on GrowlSafari.pkg and an installer wizard will guide you through the process.