The HPI Genealogy of Relational Database Management Systems v6 is now available free for download and use under the Creative Commons BY-SA license, incorporating much of the feedback I have received. Thanks! For space reasons not all systems could be included.
For corrections or to suggest additions, such as version numbers or popular RDBMS that were overlooked, please send a note to Felix Naumann at felix.naumann(at)hpi.de.