Version 1.2.1 (v125.1) of Safari, the one that comes with the 10.3.3 update, supports the document.onkeydown event, which I don't think it did before. For example, you can press Control-Shift-H when viewing mail on mail.yahoo.com to go to the Help section of the site. Or, if you're posting on an Invision Power Board, you can press Control-B to open a [b] tag. Please note that it says to press alt-B, but the correct keystroke in Safari is control-B.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20040318120332514