Numerix is a library implementing arbitrary long signed integers and the usual arithmetic operations between those numbers. It primarily targets ocaml, but is also available with reduced functionalities for the Camllight, C and Pascal languages on 32 or 64 bit Unix-type computers.
doc (en) : http://pauillac.inria.fr/~quercia/cdrom/bibs/numerix-eng.pdf doc (fr) : http://pauillac.inria.fr/~quercia/cdrom/bibs/numerix.pdf
Note: this opam package only builds the ocaml version (all modules, e.g. slong, dlong, clong, gmp and big), with the --enable-sse2 flag. Because of gmp, you need gmp headers (aptitude install libgmp-dev on debian). The --enable-longlong flag is not uses (gives some troubles on x86-64 architecture).