TOMS291, a C library which approximates the logarithm of the Gamma function.
The text of many ACM TOMS algorithms is available online through ACM: http://calgo.acm.org/ or NETLIB: http://www.netlib.org/toms/index.html.
The computer code and data files described and made available on this web page are distributed under the GNU LGPL license.
TOMS291 is available in a C version and a C++ version and a FORTRAN90 version and a MATLAB version.
POLPAK, a C library which evaluates a variety of special functions.
TEST_VALUES, a C library which tabulates several values of a variety of special functions.