The breakthrough feature in OS X version 10.4--the reason to get Tiger at all--is Spotlight. This whip-smart desktop search tool rummages through your files, folders, e-mail, and certain applications, and then displays results neatly by category.
Just like the search box in previous versions of the OS, Spotlight's search window dresses the top-right corner of any Finder window. Alternatively, pressing Command-Space summons a search field that drops down from the upper-right menu bar. The first time you use Spotlight, your Mac needs 15 to 30 minutes to index its contents. After that, the OS constantly indexes files in the background. As you would in the ITunes search window, keep typing to constantly refine your search results on the fly.
Spotlight is fast, and its searches can run deep, making it superior to previous Finder-based searches and Windows searches. Spotlight can search by the usual criteria (file name, kind, keywords) but also by a dizzying variety of deeper parameters such as audio bit-rate, codecs, color label, photo aperture, and type of encryption. It searches through files and folders, all Apple applications, and major third-party applications like Microsoft Word and Excel. Spotlight does not search through Microsoft Entourage, however, which is a bummer since not everyone uses Apple Mail.
Using my 867-MHz PowerBook, I performed a generic search for the term "PC World," which quickly produced a results list of just about every related e-mail, document, and graphic containing those words. I was even able to save my Spotlight search as a Smart Folder that continually updates and adds further PC World-related files. That means you can create a Smart Folder for each project you're working on, though you might also lose all incentive to organize your hard drive.