Slingshot has not finished yet and is under heavy development but even at this stage it looks quite nice.
To install slingshot, run following commands:
sudo add-apt-repository ppa:elementaryart/elementary-dev
sudo apt-get update
sudo apt-get install slingshot-launcher
Please note that these packages are not stable but development builds.
To launch slingshot, run the command slingshot from terminal or create a launcher. Slingshot works in both Unity and classic desktop environment.
Here is a video (Watch in HD):