Orekit is a space dynamics library, providing low-level components for the development of flight dynamics applications. On top of that it is the best designed library that I am aware of and contains beautiful code from which you can learn a lot.
I have provided some components to the library, mainly:
- Geomagnetic field models based on WMM / IGRF
- SP3 orbit file parser
- Saastamoinen Tropospheric delay model
Additionally I have served on the PMC (project management committee) for a while.