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.

Install Synapse:

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: