Others:
- GitHub - princemahajan/FLINT: Fortran Library for numerical INTegration of differential equations<
- Fortran-Astrodynamics-Toolkit/rk_module.f90 at master · jacobwilliams/Fortran-Astrodynamics-Toolkit · GitHub
- Fortran-Astrodynamics-Toolkit/rk_module_variable_step.f90 at master · jacobwilliams/Fortran-Astrodynamics-Toolkit · GitHub
There are certainly a lot of ODE solvers floating around. A good stdlib project would be to modernize, clean up and organize them all into a nice modern API.