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