The most recent version that I have been involved with is described in the following paper:
Moffat, A., Neal, R. M., and Witten, I. H. (1995) ``Arithmetic coding revisited'', ACM Transactions on Information Systems, vol. 16, pp. 256-294: abstract, associated reference.You can get software implementing the method described in this paper from this web page.
The old ``CACM'' arithmetic coding software, as published in the following tutorial:
Witten, I. H., Neal, R. M., and Cleary, J. G. (1987) ``Arithmetic coding for data compression'', Communications of the ACM, vol. 30, pp. 520-540: abstract.is available as a tar archive.
An adaptation of the CACM software to low-precision arithmetic (as described in ``Arithmetic coding revisted'') is also available [first released 8 July 1991, last updated 19 May 1993]: documentation, tar archive.