GNOME Do is basically a keystroke launcher. This means you can start typing the name of a program or a location and GNOME Do will launch it.
There are also a ton of plugins that allows you to do many other things, like quickly access your bookmarks, run terminal commands, post to twitter, use the calculator, control the sound in your system... Well, your have to see it to believe it.
Add the following to your Software Sources
deb http://ppa.launchpad.net/do-core/ppa/ubuntu jaunty mainInstall GNOME Do
deb-src http://ppa.launchpad.net/do-core/ppa/ubuntu jaunty main
$ sudo apt-get install gnome-doIt's that easy.
How to use GNOME Do
You can find GNOME Do at Applications > Accessories > GNOME Do.
To use GNOME Do you must summon it pressing Super+Space (the Super key is typically the Windows key).
Then just start typing.
Use the Down key to see the several results from what you typed.
Use the Tab key to see the options you have from the result you chose.
Check the Preferences and choose your favourite plugins.
There are also several themes, but the Docky one is the most interesting because it gives you a very nice dock.