I just noticed what might seem obvious, but no one I've asked seems to have noticed. To quickly close other open tabs in an open Safari window, simply hold the Option key while pressing the "close tab" icon (the small "X") on the tab. Very nice indeed!
[robg adds: This is an improvement over earlier Safari releases, where Option-click would close all tabs.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=2005053005373178