RT: Request Tracker
Much of the documentation for RT comes with RT itself. You can find setup instructions in the README file and see installation options by running the configure script. User documentation is under docs/. Technical documentation for each class in RT's source code is accessible using Perl's perldoc command. For example, to see the API documentation for the RT::Ticket class, run: perldoc lib/RT/Ticket.pm
This documentation is also available online:http://bestpractical.com/docs/rt/latest/
Additional information about RT and our related products is available on our community wiki. It contains helpful documentation like FAQs, troubleshooting tips, and customizations contributed by RT users like yourself.
If you're looking for more in-depth documentation, we wrote an entire book just about RT, which is available in a variety of formats.