in a nutshell this gives those developing a Plasma::DataEngines the ability to see what is being published, and people developing plasmoids can see what they can connect to. the code can be found in kdebase/workspace/plasma/engineexplorer/. the binary name is plasmaengineexplorer so as to not run into all the other engineexplorer executables out there ;)
it's not a perfect user interface by any means, but it works. and it started by hacking a bit in kdelibs. and being monday that meant compiling kdelibs several times ;)
you can also see Urs Wolfer's neat new KTitleWidget at the top. that little guy was being used quite a bit in kde4 and it became apparent that it was going to be a nightmare if we ever decided to change how it looked. so Urs whipped up a little class to do the magic. besides being used in KPageWidget, i'm already using it in krunner and the engine explorer.