Sometimes I'm viewing a site which has a collection of interesting links on it, after clicking on a link it replaces the foremost window with the new site.
The site I'm now viewing is interesting, but at the same time, I want to go back to the original site. However, I don't want to click the back button, then re-load the second site from the window in a new window/tab (this will teach me not to open all the links I want in tabs). Today I stumbled across a way of doing this in a much nicer way: just Command-click on the back button, and the first site will open in a new tab.
For example, I'm on the macosxhints.com homepage, and I then click on the first article to Read More, but I've just realised I want to see the homepage as well. So instead of clicking back and opening the second link in a new window/tab and then clicking on the second link again, I Command-click on the back arrow, and hey presto, the homepage launches in a new tab.
This works best for sites that have multiple links to other sites, and not just ones within themselves -- especially if there is a home link on the top left.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20050426115415952