MIRACL
    (Shamus Software, Dublin, Ireland)

Integer and fractional multiple precision package.
MIRACL is a portable C library. Full C/C++ source code included 
(In-line assembly support for 80x86). Number theoretic primitives
needed to support PK Cryptography are supplied.   
C++ classes for Multiprecision Integers, Modular arithmetic, and 
Chinese Remainder Thereom. Implementation in C/C++ of all modern 
methods of Integer Factorisation, viz Brent-pollard, p-1, p+1, 
Elliptic Curve, MPQS.  Includes TEX manual and some DOS .EXEs.

Not public domain, but free for academic and non-commercial use.