Synapse is a semantic launcher written in Vala that you can use to start applications as well as quickly find and access relevant documents and files by making use of the Zeitgeist engine.
Ubuntu user can install Synapse from Software Center.
For the latest version, open terminal from the dash and execute:
sudo add-apt-repository ppa:synapse-core/ppa sudo apt-get update sudo apt-get install synapse
After installation, launch synapse and you can click Active in the indicator menu to launch search box.
Or in Preferences window set a shortcut key: