http://giallo.sourceforge.net/
Giallo is a C++ library for asynchronous network programming, based on proactor style notification, independent of underlying OS demultiplexing methods. The aim is to get this accepted into Boost.
http://asio.sourceforge.net/
asio is a cross-platform C++ library for network programming that provides developers with a consistent asynchronous I/O model using a modern C++ approach.