Synapse - Synapse is a semantic launcher written in Vala that you can use to start applications as well as find and access relevant documents and files by making use of the Zeitgeist engine.
Everything by Voidtools - Everything. Locate files and folders by name instantly. Everything. Small installation file. Clean and simple user interface.
Keypirinha - A lightning fast and flexible keystroke launcher for Windows. No installation required (portable).
DocFetcher - DocFetcher is a portable German/English open source desktop search application.
Gnome Do - Simple, sleek, swift, smart. Do. GNOME Do allows you to quickly search for many items present on your desktop or the web, and perform useful actions on those items. GNOME Do is inspired by Quicksilver & GNOME Launch Box.
X1 Search - X1's Lightning-Fast Search Software. Premium Alternative to Windows Desktop and Outlook Search.