Double-double precision
Fortran 90 code by Claire Moreau-Finot for double-double precision arithmetic. That is about a hundred bits of precision with a floating point notation. The library computes the exponential function radix 2 and e, the cosine and the sine.
FMLIB Multiple precision package
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.
Large integers module
Extends the range of integers beyond 2^31. Could be useful for finding prime factors of large integers.
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.
Multiprecision Software Directory
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.
Quadruple precision for the NAS FortranPlus compil
Using the 10-byte format available with Intel processors, this module gives about 38 decimal digits accuracy.
Variable Precision Arithmetic
Lawry Schonfelder's variable precision package, using many of the new features introduced in Fortran 90.