This page gives an introduction to Arithmetic Coding and shows how to implement it using floats or integers. There is also a proof of the efficiency of the algorithms, along with visualization and Win32 binaries. This page is in English and includes links to material in both German and English.

DataCompression.info user Juergen Abel found the site quite good: Clear description, especially the explanation of the renormalisation part, full source code.

http://www.bodden.de/studies/ac/