If you have used plasmoidviewer, plasamengineexplorer or plasmawallpaperviewer from past releases of KDE Workspaces while developing Plasma components, you may be surprised to find them gone in the upcoming 4.10 release.
Well, they aren't actually gone: they have been moved to the plasmate git repository. This is where the streamlined Plasma add-ons creator has lived in nearly perpetual alpha/beta for longer than I care to admit. It's finally in the release review process (aka "kdereview") and we decided to take this time to remove all the bits that are only interesting to developers from kde-workspace and put them in one place.
This also opens the way for us to share more code between these developer tools, such as the Plasmoid previewing that is in both plasmoidviewer (obviously :) and Plasmate.
If you are concerned that you use plasmoidviewer to run individual Plasmoids in a window, do not despair! You ought to be using plasma-windowed which does a much better job of this and which has been available in kde-workspace for quite a few releases by now.
So now if you clone the kde:plasmate repository you will find all these developer tools in one convenient place. Enjoy :)