FortressInstaller
FortressInstaller is an FOSS installer project, primarily developed and maintained by The Aridity Team. It is licensed under the MIT license and is made to install/update Sourcemods using the LibGit2Sharp library. (This technically means that it only supports Git for installing and updating mods.) It was widely used in the Beta Fortress Client codebase.
Third-party dependencies
All of the third-party dependencies are licensed under the MIT license. Here is a list of the libraries that made FortressInstaller possible: