Synapse is a graphical launcher enabling you to start applications and also to quickly find and access relevant documents and files (by making use of the Zeitgeist engine).
This can be a good alternative to Ubuntu Unity Dash for those who prefer lightweight and faster graphical launcher. This can also be helpful for getting Unity Dash like feature on other Ubuntu versions like Xubuntu.
Synapse can be installed from Ubuntu Software Center. Or, by typing the following commands on terminal:
sudo add-apt-repository ppa:synapse-core/ppa
sudo apt-get update
sudo apt-get install synapse
More details can be found here: http://www.omgubuntu.co.uk/2010/11/synapse-gnome-do-launcher-app-review-ubuntu
Hope it helps.
Thanks.