If you're not satisfied with the power or flexibility of the Mac's built-in Spotlight search, you should check out HoudahSpot. This app isn't a Spotlight replacement, but rather a more versatile front-end for Spotlight--as it actually uses the technology behind Spotlight to help you find files on your computer. HoudahSpot works in much the same way as Spotlight, and you can pull it up with a key combo or a menubar icon. Once you do, though, HoudahSpot offers many more search options--and it waits until you've finished forming a query rather than immediately returning results as you type (which can be one of the more frustrating aspects of Spotlight).
HoudahSpot can be intimidating at first because it offers so many search options, letting you use multiple criteria (for example, looking for text within a file, and/or certain types of files, sizes, date ranges, etc.). You can also define the folders and directories where you want HoudahSpot to look (and not look), and you can perform basic Finder functions from within HoudahSpot (like previewing, copying, moving, duplicating, and trashing files). The best part is that you can save frequently used searches as "templates," and you can even search within your results to filter out precisely the files you need. HoudahSpot also includes nice extras like a built-in "Long Lost Files" search, a Spotlight-like BlitzSearch, and integration with third-party apps like LaunchBar. Unfortunately, HoudahSpot is currently only available for Mac OS X Leopard or later (although version 1.5.1 is available as a freeware legacy version). Users new to HoudahSpot would do well to watch the video tutorial, read the "Tips and Tricks," and visit the developer's responsive forums.