GMP is a free library for arbitrary precision arithmetic, operating
on signed integers, rational numbers, and floating point numbers.
There is no limit to the precision except the ones implied by the
available memory in the machine GMP runs on. GMP has a rich set of
functions, and the functions have a regular interface. 

GMP is designed to be as fast as possible, both for small operands
and for huge operands. The speed is achieved by using fullwords as
the basic arithmetic type, by using fast algorithms, with carefully
optimized assembly code for the most common inner loops for a lot of
CPUs, and by a general emphasis on speed (instead of simplicity or
elegance). 

GMP is believed to be faster than any other similar library. The
advantage for GMP increases with the operand sizes for certain
operations, since GMP in many cases has asymptotically faster
algorithms. 

WWW: http://gmplib.org/