Gnome Shell is the new core user interface for Gnome 3 which provides switching windows and applications but you've probably already heard of it by now so I'll skip the introduction.

GNOME Shell 2.29.0 has a lot of new features and improvements, most notable being a new message tray showing notifications sliding into the bottom of the screen, a status area for past notifications, the ability to set your presence to the user status menu, switching the overview between a grid and linear view of workspaces.