I just installed Gnome Do on my Ubuntu Hardy and I am impressed. Gnome Do is inspired by Quicksilver.
$ sudo aptitude install gnome-do gnome-do-plugin-rhythmbox gnome-do-plugins
I love the fact that you can open apps and stuff right from the comfort of your keyboard. This is an immense time saver and it is a must-have for an serious Ubuntu user.
After installing, remember to read the user guide.
Tip 1: Get gnome-do to run at startup by going to (Menu)System->Preference->Sessions and adding the command gnome-do -q
Tip 2: I didn't like the shortcut "Super + Space" so I changed it to "Control-Space" as the control key was the left most button. You can do this by opening Configuration Editor (run gconf-editor) and changing the key binding to "<control>space" in "/apps/gnome-do/preferences" inside gconf-editor.