Lcrzo network library  
   Network library for Linux, Windows, FreeBSD and Solaris providing sniff, spoof, packet creation, packet decoding, packet printing, and address conversion. [Open Source, Free]
   http://www.laurentconstantin.com/en/lcrzo/

EzSocket C Socket Library  
   EzSocket provide an easy way to write server and client programs that make use of stream sockets. [Open source, LGPL]
   http://ezsocket.sourceforge.net/

ColdFusion Socket Tag  
   A Java component for accessing sockets via ColdFusion.
   http://www.cfdev.com/products/productdetail.cfm/id/1013/

Xb2.NET  
   Object-oriented Internet library for Xbase++. Includes classes for TCP/IP socket communications, HTTP, SOAP, SMTP, POP3, FTP. [Commercial]
   http://www.sqlexpress.net/xb2net/

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.
   http://www.htthost.com

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.
   http://www.pinpi.com/

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).
   http://www.w3.org/Library/

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.
   http://www.httrack.com

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]
   http://www.melikyan.com/ptypes/

Network library netwib  
   Netwib provides client, server and most functions needed by network programs.
   http://www.laurentconstantin.com/en/netw/netwib/

CAOPI  
   A socket based communications library for use in a multi-tiered environment. [OPen source, LGPL]
   http://www.netbytes.com/caopi

Simple Sockets Library  
   Sockets library for UNIX platforms that looks much like regular C-style i/o. [Open Source, Free]
   http://mysite.verizon.net/astronaut/ssl/