Wikipedia
MLDonkey
MLDonkey is an open source, multi-protocol, peer-to-peer file sharing application that runs as a back-end server application on many platforms. It can be controlled through a user interface provided by one of many separate front-ends, including a Web interface, telnet interface and over a dozen native client programs.
Originally a Linux client for the eDonkey protocol, it now runs on many flavors of Unix-like, OS X, Microsoft Windows and MorphOS and supports numerous peer-to-peer protocols.
It is written in OCaml, with some C and some assembly.