KDE 4.0.0 was released on Friday. For the KDE team, it represents a huge leap forward in many areas, one of them being that they didn't add a K onto the name of every new app. For once. But elsewhere, KDE 4 brings huge enhancements both on and under the surface.
The panel (aka the Kicker) has been replaced by a combination desktop/panel/widget layer called Plasma. Dolphin now handles file management, leaving Konqueror as the Web browser. The entire theme was redesigned from the top down through the Oxygen project. There are a lot of important changes that will shape KDE for years.
But before you get to the good part, you have to install KDE 4. And it's a beast to install.