The Unity WebApps feature has been available in Ubuntu 12.10 for some time, but besides the Amazon and Ubuntu One Music Store webapps which are installed by default, no other webapps were available for installation.
Today, the webapps that were available in the preview PPA are available in the official Ubuntu 12.10 repositories, but as separate packages so you can install only the webapps you want and use.
Using these web applications, you get tight desktop integration for popular websites like Gmail, Google Plus, Last.fm radio, Facebook, Google Docs and many others. By desktop integration, I mean not only notifications and an icon on the Unity launcher, but also HUD, ALT+TAB, quicklists, Sound or Messaging Menu support.