In Lion Safari, you'd see an RSS button whenever you landed on a web site that had an RSS feed. This was a great way to quickly subscribe to that feed: just click the button and your newsreader would open and you could add the feed as a subscription.
Safari in Mountain Lion does not have that button, but developer Daniel Jalkut of Red Sweater Software has created a Subscribe to Feed Safari Extension. As Jalkut describes it, this extension "adds a handy button to the toolbar that, when a page offers RSS or Atom feeds, can be clicked to easily open the feed:// link, which should automatically open your favorite news reader."
Jalkut says that it's a beta, but it seems to work just fine in my tests.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20120726001105338