luabind
Luabind is a library that helps you create bindings between C++ and lua. It has the ability to expose functions and classes, written in C++, to lua. It will also supply the functionality to define classes in lua and let them derive from other lua classes or C++ classes. Lua classes can override virtual functions from their C++ baseclasses. It is written towards lua 5.0, and does not work with lua 4.
libtorrent
libtorrent is a C++ library that aims to be a good alternative to all the other bittorrent implementations around. It is a library and not a full featured client, although it comes with a working example client.