The OTRS website with source code, documentation and news is available at http://www.otrs.com/. Here you can also find information about commercial services and trainings from OTRS Group, the creator of OTRS.
The OTRS developer API documentation is available at http://otrs.github.io/doc/api/otrs/stable/.
The OTRS developer mailing list is available at http://lists.otrs.org/.