Windows: Pipy is an application launcher that combines the best of apps like Launchy with command-line features from Unix-based systems. It comes with useful file search functions that your computer might be missing.
Pipy does all of the basics seamlessly: launch it with a key-command and start typing to launch applications, or search your computer for files and folders. Commands such as "search" and "launch" do what they're supposed to do, and "websearch" queries your preferred search engine. Pipy really shines when you want to do sequential searches or filter search results. For example, if you're searching for files named "Lifehacker" that are also mp3s, just type "search Lifehacker | grep mp3" and the app will show you the results immediately.
Pipy has a customisable interface, and you can change your preferred search engine and settings from the app preferences. It's a little rough around the edges, but it's definitely worth a try. It's completely free and Windows only.