Desktop Search for OS X
Alfred (OS X)
Alfred is a combination app launcher and desktop/web search utility for OS X. Once installed, Alt+Space brings up a search bar that you can use to lauch any application on your system, search the web, and of course, search your system for files, folders, images, and more. You can add and remove folders from Alfred's search scope, and use its built-in engine to search for documents, or you can turn on Alfred's search "Extras" to make it index and present file and folder results as you type. It's a little slower (especially when you can press space again after bringing up Alfred's search bar to look for files) but it gives you everything up front. The beauty of Alfred is that it's an app launcher and desktop search tool in one, but it's also much more. Alfred sports an iTunes mini-player, built-in calculator, support for customizable hotkeys, and has a wealth of available extensions to add even more features. It's free, although the Alfred Powerpack will set you back £15 (approx $23 USD) and toss in some extra useful features.
Quicksilver (OS X)
Quicksilver has a long and storied history. For a while, it was the best app launcher and customization tool available for the Mac. We loved it then, and when its future looked a little grim, we mourned. Still, it went open source, and when independent developers picked it up, we were happy. Since then it's been updated frequently, and is still our favorite app launcher for Mac. Even though it's an app launcher, it's also a great file and document search tool, and allows you to browse and search your Mac's file system quickly and with a few keystrokes. Right out of the box, Quicksilver lets you search files, folders, documents of all types, contacts, bookmarks, and more. It can chain commands, so it can search for a file, then give you options of how to open it, or find a file and then move it to a file location for you. It also has a wealth of independently created plugins that can extend its search and scripting features, which let you really play with what the app can do under the hood. The latest versions of Quicksilver are really sharp, and it's completely and totally free (although the team behind it definitely appreciate donations to keep the project running.)
Launchy (Windows/OS X/Linux)
Launchy is our favorite application launcher for Windows, and as we pointed out when we explained why you should be using an app launcher, it can do much more than just find and launch the apps you want to use when you want to use them. Launchy is super-fast, cross-platform, and makes finding files, launching applications, and chaining activities (finding files and then opening them in a specific application, or searching for a term and then opening it in Google in your favorite browser) really simple. It needs a little time to index before it's really effective, but you won't notice it working in the background. Launchy stays lean and trim by cutting out the bloat and other added features that other app launches include, but for the purposes of desktop search, it works like a charm and is super-simple to use. It's completely free, and works just as well on OS X and Linux as it does in Windows.