I'm not sure if this has been posted before, but a quick search didn't turn up any matches, so here's the hint...
If you select some text in Safari and then press Command-Shift-L, Safari will open a new tab (or window, depending on if tabs are enabled in your preferences) with the Google search results for your selected text.
[robg adds: An earlier hint covered Google-searching on selected text via the contextual menu, but didn't mention a keyboard shortcut. I check Safari's Keyboard shortcut list, and command-shift-L is shown as blank, so this appears to be a hidden shortcut...]
[robg apology addendum: As has been pointed out in the comments, this is just Safari's built-in Search service -- sorry for the "obvious hint" posting ...]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20040223102323686