C++ Portable Types Library (PTypes) - - A simple alternative to the STL plus portable multithreading and networking. Compiles under modern Unix and Windows systems. [Open-source, zlib-like license.
EzSocket C Socket Library - - EzSocket provide an easy way to write server and client programs that make use of stream sockets. [Open source, LGPL.
HTTPort - TCP/IP through HTTP tunneling software - - With the HTTPort SDK you can make the Internet software you write working from behind an HTTP proxy. Demo version includes binary code and samples in C, Visual Basic and Delphi.
HTTrack Website Copier - - HTTrack is a free (libre/open source) and easy-to-use offline browser utility. Source code is available for Windows and Linux/Unix/BSD.
Libwww - the W3C Sample Code Library - - Libwww is a highly modular, general-purpose client side Web API written in C for Unix and Windows (Win32).
Network library netwib - - Netwib provides client, server and most functions needed by network programs.
Platform Independent Network Programming Interface (PINPI) - - PINPI is a programming interface based on an all-purpose platform independent networking protocol and data format (SDXF, the Structured Data eXchange Format)for use to realize client/server (or similar) applications.
ProstoServer - - Universal library for building high-performance TCP servers running under Microsoft Windows.
Simple Sockets Library - - Sockets library for UNIX platforms that looks much like regular C-style i/o. [Open Source, Free.
Xb2.NET - - Object-oriented Internet library for Xbase++. Includes classes for TCP/IP socket communications, HTTP, SOAP, SMTP, POP3, FTP. [Commercial.