DCOP
KDE 2 and 3's own message bus, the Desktop COmmunication Protocol, which let every KDE program be driven from the command line and which D-Bus later copied.
KDE had planned to build its second version on CORBA, as everyone in the late 90s assumed one should, and gave up when it turned out every component needed IDL files, an ORB and patience. Matthias Ettrich and Preston Brown instead wrote something small on top of ICE, the library X11R6 already used for session management. One server per session, dcopserver, kept track of which programs were connected; each program published objects with functions, and a call was addressed by application name, object and signature. Arguments were marshalled with Qt's own data types, which was easy as long as both ends were KDE programs, which was the whole point.
What made DCOP loved was the dcop command: you could change the wallpaper, drive the music player or open a tab in Konqueror from a shell script, and kdcop let you browse everything that could be called. The model was good enough that Havoc Pennington took it as the template for D-Bus, which KDE 4 switched to in 2008. It was one of the rare occasions when the GNOME side copied KDE and KDE then took the result back.