x11vnc

Karl Runge released x11vnc in 2002 as part of the LibVNCServer project. The usual VNC servers for Unix, Xvnc and its descendants, are X servers in their own right with no physical screen: you get a fresh desktop that exists only in the remote session. x11vnc does the opposite. It connects as an ordinary X client to display :0, reads the framebuffer and ships it out as RFB. It finds changes with the Damage extension or by scanning rows, injects keystrokes and mouse movement through XTEST, and picks up the cursor via XFIXES. The result is what Windows Remote Assistance or Mac screen sharing offer: the person in front of the machine sees what the guest is doing.

That made it the tool for support calls and for reaching a home desktop you forgot to log out of. Runge kept adding: SSL, a Java viewer served from a built-in web server, and a graphical front end, tkx11vnc, whose many tabs mostly demonstrated how many flags the command had accumulated. His own development stopped in 2011 and the code has been maintained within LibVNCServer since. Wayland has no equivalent, because a compositor does not let an arbitrary client read the whole screen.