By accident, I clicked a link in Safari instead of command-clicking it to open it in a new tab. However, whilst it was loading, I command-clicked the link again, and it stopped loading in the current tab and opened in a new tab instead.
[robg adds: I cannot duplicate this behavior, though the author has confirmed it, and it works on a friend's machine. I'm not sure if it has to do with the speed of the connection or something else, but it simply doesn't work on my machine...]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20030418131458629