After I updated to Safari 3, AcidSearch stopped working properly, so I uninstalled it. I got Inquisitor instead, but I missed the ability to do a Google site search on whatever site I was visiting. So I put together this AppleScript, mostly assembled from other people's scripts:
[robg adds: To use this, save it as a Script in ~/Library/Scripts/Safari, and make sure the Scripts Menu is enabled. Then just select it while visiting any page in Safari, enter the search term in the box, and you'll run a Google search restricted to that site. It worked fine for me when testing.]Mac OS X Hints
http://hints.macworld.com/article.php?story=20071201151055199