Here are some open source projects I’ve worked on related to rocketry:

Rocketbeacon

Sometimes referred to unofficially as an “Elvin beacon” by discord users.

This is a small and highly configurable radio beacon. It can be used with a directional antenna and a handheld radio as a low cost option to find rockets. The design files are freely available, and boards can be ordered from JLCPCB. If you have the space, a GPS tracker combined a radio beacon is nice in terms of redundancy. If you ask around on the /r/rocketry discord, you’ll find others selling soldered and configured units ready for use.

OpenChute

A program for designing parachutes. It supports arbitrary cross-sectional parametric geometries and can be used to construct many different parachute types including toroidal, elliptical, disk-gap-band, and ringsail parachutes:

SoliDAQ

A self-contained data acquisition board for solid rocket motors. It uses an AD7124 and supports load cells, 4-20 mA pressure transducers (with 12V excitation), and thermocouples. It has been used successfully for static testing, but the firmware is not really “production-ready”