Open Rocket is an excellent open source project and the software enables you to design a rocket and simulate its flight using a variety of materials and a variety of commercially available motors.
Poliastro is an open source (MIT) collection of Python functions useful in Astrodynamics and Orbital Mechanics, focusing on interplanetary applications. It provides a simple and intuitive API and handles physical quantities with units.
Whilst this active group aren't fully open source, they do have a great collection of tools and projects on their Git repositories. They have numerous hybrid designs and other amateur high power rocketry projects. There are many design tools on the GitHub page and a great collection of technical blog posts on various rocketry aspects.
The Flame Trench have released the designs for the NanoAlt family of sport altimeters on github. This project is not complete and needs the software aspects developing, the nanoAlt looks like an excellent option for competition rocketry as it will fit in tiny bt5 based rockets.
Team ZPC a group of Aeronautical Engineering at UTN- Haedo, Argentina, have released this python based project that aims to help people tune their PID's for rocket TVC systems.