Copernic Desktop Search
Much, much better than any I've seen until now on the Windows platform.
Copernic — I tried their tool in the late 90s, when it was a desktop version of popular search engines that collated results and categorized them (ala Vivisimo now). It's one of those things you try and then give up.
But they sure have come a long way since then. If you read this site you know that I've tried and tested a bunch of desktop search tools from Google to 80/20 to Isys.
But I have no hesitation in saying that Copernic's desktop search tool is among the best there is. (Have been pending this entry because I want to include screenshots, but that'll come later.)