CTorrent is a BitTorrent client implemented in C++ to be lightweight and quick.
The original has fallen a little behind in updates and bug fixes though.

The files here contain the good work of those who wrote the original CTorrent
base code and a number of patches that provide fixes and enhancements, as well
as additional fixes and enhancements.

The purpose of the Enhanced CTorrent effort is to fix problems that remain in
the code, modernize existing features and algorithms, and implement new
features while maintaining low overhead requirements and a high standard of
performance (both part of the original CTorrent design philosophy).
Highlights of the enhanced client include:

* Support for large files (>2GB) and large torrents (>255 files)
* Strategic selection of pieces to request for download
* Continuous queueing of download requests, tuned based on latency and
  throughput for each peer
* Improved download performance, including parallel requests in initial and
  endgame modes
* Improved bandwidth regulation
* Improved compatibility with other peers
* Performance optimization and bug fixes
* An interface for monitoring and managing multiple clients
* Dynamic cache allocation and management, including prefetch
* Flexible console I/O redirection
* Interactive control commands and menus