KDE/Plasma/KWin 4.x that ships in Ubuntu 14.04 has one amazing feature that I discovered today. I keep many XTerm, RXVT and related file explorer windows open. These were related to many servers I was SSHed to and I was looking for a way to group windows related to each server together. Guess what, KDE 4.x has a fantastic solution!
KDE 4.x allows you to group any set of open application windows together as tabs. Just right-click on the titlebar of an application and choose Attach as tab and pick the other application with which you want to group this one. These two application windows will now appear in a single window as tabs! You can repeat this operation and group together apps as tabs inside super-windows. (In older KDE, this option appears as Move window to group.) This works for any application window, they can be from different applications. What a great way to organize your windows on the desktop!
Sadly, this feature is missing in the recent KDE/Plasma/KWin 5.x series that ships in Ubuntu 16.04. It lies on the missing features list and I really hope they will reimplement this great feature.