RED is a stand-alone requirements engineering tool. It is based on Eclipse and offers a great variety of modeling concepts and features.
RED is not (yet) open source, but it is free to be used for whatever purpose you see fit.
Including diagrams in a report will demand to press "Cancel" once (!) for each included diagram.
Cascade Delete does not cascade to diagram elements.
Using RED on Macs, opening diagrams sometimes derails the whole process, sometimes it just consumes disproportionate amounts of ressources, but comes back, or stays responsive.
Cascade Delete does not cascade to diagram elements.