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.

Scherlokk - Mac OS X application. Simple and user friendly, Scherlokk will find your media and missed files - quickly.

Keypirinha - A lightning fast and flexible keystroke launcher for Windows. No installation required (portable).

Slingscold - Slingscold is a lightweight application launcher similar to Launchpad in Mac OS X.

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.

kupfer - Kupfer is a command tool heavily inspired by https://alternativeto.