Debug Plugin functionality has been merged into XPages SDK for Eclipse RCP project. The download link is not relevant anymore.

This plugin can be used by XPages developers writing code using the Extensibility API from an Eclipse IDE to debug code without having to deploy it first. See here for the documentation.

Download the release from here.

Starting in IBM Lotus Domino Server 8.5.2, XPages provides a toolkit for writing custom controls using OSGi plugins. Developers need to export their custom plugins from Eclipse and install them on the Domino Server file system before they can run and be debugged. These operations must be repeated every time a change is made in the code.
To improve productivity, the IBM Lotus Domino Debug Plugin removes the need for exporting and deploying the custom plugins by configuring the IBM Lotus Domino OSGi runtime to load the plugin classes directly from the Eclipse workspace.

David Taieb is the developer, Niklas Heidloff only the committer.