CAOPI
A socket based communications library for use in a multi-tiered environment. [OPen source, LGPL]

ColdFusion Socket Tag
A Java component for accessing sockets via ColdFusion.

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.

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]

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.

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]