Pull requests and forks, you can fork a project and keep it up to date with the master. People's hacks are not included in the source code, they're free to for

Pull requests and forks, you can fork a project and keep it up to date with the master. People's hacks are not included in the source code, they're free to fork the project and implement their own hacks and submit a pull request to get their solution included as part of the master if it is approved