A couple of years back there were no contenders for this title. Today we have a wealth of choices. In a close race some one would have to say Copernic Desktop Search [1]. It's a very competent and balanced product and one of the few freebies that supports network shares. The only beef is the presentation of email search results is not as effective as other search engines such as X1, a product that is unfortunately no longer available in a free version (although it is integrated into free email client Eudora's find function)
Another option is Google desktop search [2]. It not only will search your hard drive files but also your web history. It offers an Outlook toolbar, integrated Gmail search and a novel desktop sidebar that allows personalized search, news, weather, photos and more. The Sidebar also includes a quite effective application launcher. Some folks love the Sidebar but others, me included, find it intrusive. My main problem with Google Desktop search is again the presentation of email seach results which is even poorer then Copernic.
This is not an academic point. For many users searching email is the number one application for desktop search programs so you need a product that performs well in this area.
Exalead [3] is a little known but highly impressive desktop search program from France. It's great strength is the highly customizable nature of its search, probably the best of any product. It can also do fuzzy searches as well. If you need to make highly specified searches then do try Exalead, you wont be disappointed. Versions are available for English and French speakers.
Ava Find [4] is a strong competitor. The first time it starts up, this substitute for Microsoft Find scans your drives and stores the locations of your files in a cache. Once completed, it delivers results instantly as you type. You can limit search results by folder or drive and specify whether you're looking for videos, music, applications, or recent downloads. However, it requires a lot of computing power; and, after 30 days, Ava Find won't let you delete, rename, or perform most other file functions from the results window. Those essential features are limited to the Professional version.
[1] http://www.copernic.com/en/products/desktop-search/index.html Freeware, Windows 98 and later. 3MB
[2] http://desktop.google.com Freeware, Windows 2000 SP3 or later, 2MB
[3] http://corporate.exalead.com/enterprise/l=en?p=produits_exalead-desktop_screenshots Freeware, Windows 2000 or later, 8.7MB
[4] http://www.think-less-do-more.com/avafind/ Free. Includes 30-day preview of Ava Find Professional, Windows 2000, Windows XP, or Windows 2003 Server, 950 KB