I was checking my RSS feeds in Safari, and when I tried to click a link in my bookmark bar, I accidentally clicked on the blue RSS logo in the location bar in Safari instead.
The RSS page then rolled up in a neat animation, revealing the associated website's main page. Clicking the RSS logo button again lowers the RSS window back down. Neat trick.
[robg adds: In my testing, it seems that clicking the logo hides the RSS feed and reveals whichever page you had open underneath it, which is not necessarily the website's main page.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20061001141514923