SOFA
The Standards of Fundamental Astronomy (SOFA) library is a very nice collection of routines that implement various IAU algorithms for fundamental astronomy computations. Versions are available in Fortran and C. Unfortunately, the Fortran version is written to be of maximum use to astronomers who frequently travel back in time to 1977 and compile the code on their UNIVAC (i.e., it is fixed-format Fortran 77). To assist 21st century users, I uploaded a little Python script called SofaMerge to Github. This script can make your life easier by merging all the individual source files into a single Fortran module file (with a few cosmetic updates along the way). It doesn't yet include any fixed to free format changes, but that could be easily added later.