Update 07/01/2015 Synapse is no longer officially maintained (it's not even available on Ubuntu repositories since version 14.04) so its stability is not assured.
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.
Open a terminal window and run:
sudo apt-get install synapse -y
Synapse can be accessed via Applications Menu > Accessories > Synapse or typing the following command:
Synapse runs on the background and can be activated using the keyboard combination
Ctrl+Space. This combination and other settings (like add to startup) and be altered accessing the preferences dialog: