This hint is possibly well known, but I just came across it in a newsletter from OWC:
By adding the suffix site:www.XYZ.com to your search terms, Google knows to limit its search to that particular site. Replace www.XYZ.com with the site to which you are attempting to limit the search.
[robg adds: No, this isn't an OS X hint. Given the popularity of Google, however, I thought I'd run it. For a full page of tidbits on getting the most out of Google, read their expert search tips.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20030328081030703