Msieve is a library and utility for factoring large integers using the most
powerful modern algorithms.  It features a stable and very fast implementation
of a self-initializing multiple polynomial quadratic sieve (MPQS), plus a
somewhat experimental general number field sieve (GNFS) implementation.

Primary design goals are speed, portability and ease of use.  Msieve claims to
be the fastest implementation for factoring general inputs between 40 and 100
decimal digits, but can handle larger input as well.

WWW: http://www.boo.net/~jasonp/qs.html