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Starting with Tiger, Safari has been able to display RSS feeds, which many blogs, news sites, and other online publications use to distribute information on an article-by-article basis. But frankly, Safari was never a great RSS reader; it does the job, but not with the wealth of features or flexible interface of stand-alone RSS readers such as NetNewsWire (http://www.newsgator.com/Individuals/NetNewsWire, free). With Leopard, Apple has tried a different approach: putting RSS support in Mail.
In principle, this makes more sense. Mail's interface is better than Safari's for displaying RSS feeds, and many of the tasks you perform in Mail (marking messages as read, searching, using smart mailboxes, and so on) can be done exactly the same way with feeds. Having spent several months giving Mail's RSS capabilities a good workout, my opinion is that if I were stuck on a desert island with only Mail to meet my RSS needs, I'd manage, but I'd prefer a more capable tool.