TOMS452 is a FORTRAN77 library which implements ACM TOMS algorithm 452, for generating, one at a time, the combinations of M things out of N.
The text of many ACM TOMS algorithms is available online through ACM: http://www.acm.org/pubs/calgo or NETLIB: http://www.netlib.org/toms/index.html.
TOMS452 is available in a FORTRAN77 version.
COMBO, a FORTRAN90 library which contains several routines for generating, ranking, or randomly selecting a combination.
SUBSET, a FORTRAN90 library which contains many routines for generating, ranking, or randomly selecting a combination.
You can go up one level to the FORTRAN77 source codes.