Libwww is a highly modular, general-purpose client side Web API written in C for Unix and Windows (Win32).

HTTrack is a free (libre/open source) and easy-to-use offline browser utility. Source code is available for Windows and Linux/Unix/BSD.

Netwib provides client, server and most functions needed by network programs.

A simple alternative to the STL plus portable multithreading and networking. Compiles under modern Unix and Windows systems. [Open-source, zlib-like license]

Sockets library for UNIX platforms that looks much like regular C-style i/o. [Open Source, Free]

Object-oriented Internet library for Xbase++. Includes classes for TCP/IP socket communications, HTTP, SOAP, SMTP, POP3, FTP. [Commercial]

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.

Universal library for building high-performance TCP servers running under Microsoft Windows.

EzSocket provide an easy way to write server and client programs that make use of stream sockets. [Open source, LGPL]