mimetic
- A C++ MIME library for Unix built around the standard library. [Open Source, GPL]
- http://codesink.org/mimetic_mime_library.html

Libiqxmlrpc
- A free implementation of XML-RPC in C++. It supports multithreading, secure connections and interceptors.
- http://libiqxmlrpc.sourceforge.net/wakka.php?wakka=HomePage

Socket++
- Defines a family of C++ classes that can be used more effectively than directly calling the underlying low-level system functions. One distinct advantage of the socket++ is that it has the same interface as that of the iostream so that the users can perfo
- http://www.linuxhacker.at/socketxx

gloox
- A portable C++ Jabber/XMPP library which takes care of the low-level protocol stuff. Additionally, it offers high-level interfaces for interaction with an XMPP server. [Open Source, GPL]
- http://camaya.net/gloox

MIME++
- Class library for parsing, creating, or editing messages in MIME format.
- http://www.hunnysoft.com/mimepp/

VMime
- A C++ class library for working with MIME messages and Internet messaging services like IMAP, POP or SMTP. [Open Source, GPL]
- http://www.vmime.org/

C++ Sockets Library
- A C++ class library wrapping the berkeley sockets C API, and therefore works on most unixes and also win32. Features include, but are not limited to, SSL support, IPv6 support, TCP and UDP sockets, HTTP protocol, and highly customizable error handling. [O
- http://www.alhem.net/Sockets/

Stream Socket API
- Provides a simple stream socket interface for C++.
- http://www.pcs.cnu.edu/~dgame/sockets/socketsC++/sockets.html

SimpleSocket
- Aa C++ library making socket based client/server programming easier.
- http://home.hetnet.nl/~lcbokkers/simsock.htm

Simple Unix Events (SUE)
- A collection of light-weight C++ classes which help to create a select-based event-driven applications, especially simple TCP servers and clients. [Open Source, LGPL]
- http://www.croco.net/software/sue/

Jabberoo
- An object-oriented, cross-platform C++ library which provides handling logic for the Jabber protocol. [Open Source]
- http://jabberoo.sourceforge.net/

CSock
- Provides synchronous send and receive of a buffer, plus ping and resolve address. The class encapsulates a listener thread which can be started and stopped.
- http://www.firstobject.com/dn_sock.htm

ACE (ADAPTIVE Communication Environment)
- A freely available, open-source object-oriented framework that implements many core patterns for concurrent communication software.
- http://www.cs.wustl.edu/~schmidt/ACE.html