Beryl was a very advanced 3D desktop window manager which looked extremely fancy. It could place virtual desktops on a cube. It had wobbly windows, exploding windows and burning effects and many other effects. Beryl started off as a fork of Compiz in 2006 and, ironically, died with a merger with Compiz into Compiz-Fusion in 2009. Compiz-Fusion later dropped the -Fusion part and it still exists as just Compiz.
Linux Window managers allow you to have virtual desktops who each have their own set of applications. Beryl allowed you switch between these desktops using a rotating cube. This alone made Beryl look very impressive in 2006. Beryl's default settings gave you a four-sided cube. But it didn't have to be four-sided, Beryl allowed you to have, say, nine virtual desktops on your cube. The standard setting placed you outside the cube. It was also possible to view it from the inside. Additionally, you could make your background (the wallpaper) transparent and view the other sites of the cube in the background.
|Nine-sided Beryl cube||Inside a four-sided cube|
The cude-feature alone was a good reason to show Beryl off to friends and people stuck with proprietary operating systems without anything remotely resembling Beryl's capabilities.
Beryl could make your windows "wobble" when you moved them. It also had many fancy effects when windows are opened and closed. Basically, it's was so fancy it was almost ridiculous. This is what it looksed like when one of these effects was taking place while the cube rotates:
Beryl itself has not been a thing since it's merger with Compiz in 2009. Compiz, however, is still kind-of alive. You can have all the effects Beryl had and more by installing a Compiz. However, Compiz hasn't made a release since 2011 so it's questionable how well it will work with modern X versions and modern Mesa versions. It's website is at http://www.compiz.org/
Other modern window-mangers like KDE Plasma's KWin have many of the same effects. KWin has wobbly windows, exploding windows and a cube. It is, of course, not as fancy as Compiz but it is close.