It is very easy to create a service-menu. In a nutshell it is just a text-file that allows to start an application with the selected files as parameters.
But service-menus also have some restrictions: Only the MIME-type is used as criteria to define which entries are shown. So it is not possible to show a service-menu only if e.g. exactly two files are selected or to create dynamic service-menus dependent on some meta-data of the selected files.
Sebastian Trüg has written an interface called KFileItemActionPlugin
Some developers might notice the similarity to the KonqPopupMenuPlugin, that has been available since ages, but is not supported by Dolphin. The reason why it is not supported in Dolphin are some interface-issues when using the plugin outside the scope of the Konqueror context-menu. After discussing this with David Faure and Sebastian Trüg we decided to cleanup the interface, call it KFileItemActionPlugin and mark the KonqPopupMenuPlugin as deprecated.
Sadly there are not much applications for KDE 4 that take usage of the KonqPopupMenuPlugin interface. On one hand this is good as this means less efforts to port it to the new KFileItemActionPlugin interface. But on the other hand for the users out there this means a not so tight interaction of the KDE file-managers with applications.
So this is a kind of invitation to all application developers out there: Possibly the KFileItemActionPlugin is also a benefit for your application to get integrated in a better way into the KDE file-managers :-)