The open-source Cyberismo Secure development essentials content module includes excellent tools for threat modelling, which is one of the most important activities in secure development.

Threat modelling requires an understanding of the trust boundaries, processes, external entities, data stores, and dataflows between the entities. With Cyberismo, you can model the structure of your solution with cards and links, and the tool will generate dataflow diagrams automatically.

Besides threat modelling, you can try out the modelling tool for architecture documentation: model your architecture, create architecture decision records, and write design specifications!

As threat modelling is a part of the Secure development essentials module, it integrates very well with the rest of the secure development process.