Slingshot launcher is developed by Elementary team that is using in Elementary OS. Here is how to install Slingshot launcher in Ubuntu Natty from elementary team ppa and integrate it into Unity Launcher. See:

Install Slingshot from Elementary Team PPA

Before this, read the ppa description:

Do NOT install this PPA if you want a tested running system, this PPA contains the newest and most unstable development of elementary, this PPA is only recommended to use if you are a developer and are not afraid to encounter critial bugs.
Also this PPA may overwrite already installed packages you don’t want it to!

Some elementary apps have their own development PPAs. If you want an even more unstable system, you can add them too: (Plank)

If your make up your mind to install it, run following commands in terminal:

sudo apt-add-repository ppa:elementaryart/elementary-dev
sudo apt-get update
sudo apt-get install slingshot-launcher

Create an icon on Unity Launcher for Slingshot

Create and edit .desktop file under /usr/share/applications/ folder with the command:

sudo gedit /usr/share/applications/slingshot.desktop

Write in following into this file (change the last line Icon=path/to/your/icon):

[Desktop Entry]

Now, open up nautilus file browser and navigate to /usr/share/applications/, drag and drop the slingshot shortcut icon onto Launcher.