Unworkable is a BSD-licensed BitTorrent implementation
for UNIX written from-scratch in C. It uses libevent
for scalable asynchronous networking and the mmap()
system call for local data access. Some of the goals of
the project include (in no particular order) high code
quality, efficiency, simplicity and security.

Unworkable is still in an early stage of development,
and is far behind most other BitTorrent implementations.
However, it is usable for some basic things and the
source code is quite minimal(4,000 lines of C compared
to rTorrent's 40,000+ of C++).

WWW: https://code.google.com/p/unworkable/