David Bailey's package for multiple precision, and packages for double-double (~32 dec. digits) and quad-double (~64 dec. digits) precision. The last two are in C++ with Fortran front ends.

Multiple precision arithmetic.

This is Richard Brent's classic MP package, which was published as TOMS algorithm 524. Contains some features such as the Bernoulli numbers, which are not available in either Smith or Bailey's packages. Dates from 1981.

David Smith's package for multi-precision arithmetic. Unlike most other packages, the components of these large numbers are stored as REALs. This is usually much more efficient than storing them as integers. This is TOMS algorithm 786.