TOMS385 is a FORTRAN77 library which implements ACM TOMS algorithm 385, for evaluating the exponential integral Ei(x).
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.
TOMS385 is available in a FORTRAN77 version.
TEST_VALUES, a FORTRAN77 library which supplies test values of various mathematical functions.
You can go up one level to the FORTRAN77 source codes.