GMP is a free library for arbitrary precision arithmetic, operating on signed integers, rational numbers, and floating point numbers. There is no practical 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.
Another definition:
cristi:demos diciu$ date; ./primes 100000000 | tail ;date
Fri Aug 29 13:24:22 EEST 2008
99999787
99999821
99999827
99999839
99999847
99999931
99999941
99999959
99999971
99999989
Fri Aug 29 13:24:23 EEST 2008