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.
DocFetcher - DocFetcher is a portable German/English open source desktop search application.
Keypirinha - A lightning fast and flexible keystroke launcher for Windows. No installation required (portable).
Everything by Voidtools - Everything. Locate files and folders by name instantly. Everything. Small installation file. Clean and simple user interface.
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.
Agent Ransack - Agent Ransack is a tool for finding files and information on your hard drive fast and efficiently.