OWASP Threat Dragon
Threat Dragon is an open-source threat modelling tool from OWASP. It comes as a web application or an Electron based installable desktop app for MacOS, Windows and Linux.
The desktop app saves your threat models on your local file system, but the online version stores its files in GitHub. This means that to use web app you have to sign in with a GitHub account and give it write access to your public repos. Other than that, the user experience is currently almost identical between the web and desktop variants. In the future, there will be deeper integration with GitHub (and other code repositories).
Threat Dragon is currently in development as an OWASP incubator project, so there might still be some bugs. You can find the source code for Threat Dragon on GitHub OWASP / threat-dragon where you can also ask for changes or report any issues.