I'm not claiming this as the greatest thing since sliced bread but it is quite a nice side-effect of Safari's autofill function.
The annoying thing about sites like eBay is that you don't get an instant list of previous searches (I pick eBay because I first found this there, but it is of course generic). But if you have Safari's autofill switched on, then if you return to a page where you've already done a search, typing the first few letters of that search makes the whole text reappear.
And because Safari ties all this to the particular web page, the autofill doesn't turn up anywhere else.
Like I said, not earth shattering but it makes my life just that little bit easier. I'm running OS 10.2.6 and Safari 1.0.85...
Mac OS X Hints
http://hints.macworld.com/article.php?story=20030930115834795