Slingshot is a new application launcher created by the Elementary team. Here is a video with Slingshot in action recorded by the Elementary team:
Besides Slingshot, the video above might also be revealing the Elementary OS look - an Ubuntu-based Linux distribution that will be released in March. The video is not that great but for now I couldn't get Slingshot to work myself.
Slingshot is still work in progress and there are not downloads available, but if you want to give it a shot, get the code via BZR.
Slingshot is not the only new addition to Elementary OS as it seems a lot more applications are on the way: a backup app called Sassy Backup, Elementary widgets, Elementary Gedit, Maya calendar and probably more. Further more, it seems some of these might be incorporated into Slingshot (at least that's how it appear by looking at the code).