TOMS443 is a C library which evaluates Lambert's W function. This is a version of ACM TOMS algorithm 443, by Fritsch, Shafer and Crowley.
Lambert's W function W(X) satisfies the equation
W(x) * exp ( W(x) ) = 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.
TOMS443 is available in a C version and a C++ version and a FORTRAN77 version and a FORTRAN90 version and a MATLAB version.
TEST_VALUES, a C library which contains routines which return sample values of various functions, including the modified beta function, and the logarithm of the gamma function.
TOMS743, a C library which evaluates Lambert's W function. This is a version of ACM TOMS algorithm 743, by Barry, Barry and Culligan-Hensley.
You can go up one level to the C source codes.