Free Internet, Email, HTTP, Sockets / Winsock Programming Source Code

Qpopper POP3 Server
Qpopper, from Qualcomm, is a POP3 server for Unix/Linux that comes with source code. It supports TLS/SSL.
Cucipop POP3 Server
This is a POP3 server that comes with source code. I do not have any more information about it.
TurboPower Internet Professional (iPRO)
TurboPower Internet Professional is a set of VCL components (which means that you need to use Borland Delphi or Borland C++Builder to use it) that allows your program to access the Internet in numerous ways. In terms of email, it supports POP3, SMTP, and NNTP, attachments (encoding and decoding), MIME, etc. It also handles Instant Messaging (IM), multiple HTTP requests at the same time, rendering of HTML with support for PNG, JPEG, BMP, WMF, ICO (etc), non-visual HTML scanners (for writing web spiders and robots), logging, data conversion, address lookups, Remote Access Server (RAS) support, VT100 terminal emulation, Internet Control Message Protocol (ICMP) components (for ping and trace), FTP (including an FTP directory tree component), SNTP (for time clients/servers), troubleshooting protocols components (Echo, Discard, CharGen), client/server components for finger and whois, etc.
TurboPower Async Professional (FTP, VOIP, etc)
If you need to add speech to your programs, or enable it to process speech input, TurboPower's Async Professional provides you an easy access to Microsoft's Speech API (SAPI) allowing you to integrate voice/speech recognition and speech synthesis in your programs. It also handles the sending and receiving (as well as viewing and printing) of faxes (and converts BMP, DCX, PCX and TIF files automatically), alphanumeric and text (GSM/SMS) paging, high speed file transfers (including the major file transfer protocols like Zmodem and FTP), IP telephony (audio and video, Voice Over IP or VOIP), Internet communications (Winsock access), supports ISDN modems, RS-232 and RS-485 standards, and includes an XML-based modem database. And so on. Like most of the TurboPower tools, this open source toolkit may be used with Borland Delphi and Borland C++ Builder and ActiveX environments like Microsoft Visual C++ and Visual Basic. If you use Borland Kylix on Linux, you might want to try the TurboPower Async Professional CLX version.
TurboPower SysTools (with email MIME encoding/decoding)
TurboPower SysTools contain numerous utility functions and classes for Borland Delphi and Borland C++ Builder as well as programming environments that support COM objects (like Microsoft Visual Basic, Microsoft Visual C++. Among the routines included in this library are string manipulation routines, date and time math routines, high-speed sorting, high-precision maths, runtime math expression analyzer, Microsoft Excel-like finance and statistical routines, email attachment MIME encoding and decoding, reusable containers (stacks, queues, trees, deques, hash tables), CRC data integrity checking, 1 dimensional and 2 dimensional (1-D and 2-D) bar codes, money routines, logging classes, patterns, random number distributions, text data manipulation and more. SysTools is open source, released by TurboPower under the Mozilla Public License. It works on Windows systems. There is also a version of TurboPower SysTools for Kylix.
The Standard Function Library
The Standard Function Library, or SFL, contains routines for compression, encryption, encoding, data type conversion, data output formatting, process control, dates, times, calendars, symbol tables, linked lists, configuration files, error message files, string manipulation, searching, Internet socket access, MIME, CGI, SMTP, batch programming, debug tracing, directory access, environment access, etc. It is designed with portability in mind. The library is written in ANSI C and is supported on MSDOS, Windows, OS/2, Linux, AIX, SunOS, HP/UX, Solaris, NetBSD, FreeBSD, SCO OpenServer, Digital Unix, and OpenVMS. The source code is also available.
Putty: Win32 SSH / SFTP Client with Source Code
Putty is not a library, but a set of programs that implement Secure Shell access, Secure FTP (SFTP), Telnet, etc. Since it is open source, employing things like strong authentication and secure sessions, it may be a useful source of information for your own internet software.
OpenSSL
If you are writing an application (browser, email, server, etc) that requires the use of either Secure Sockets Layer (SSL versions 2 or 3), or Transport Layer Security (TLS version 1), you can obtain this free open source "commercial-grade" OpenSSL library. The library allows you to add cryptographic functionality to your programs, apparently both server programs as well as SSL supporting clients. You can use it in both commercial and non-commercial programs.
OpenSSH
This is not a library but an open source suite of programs that support Secure Shell access (SSH), namely ssh (which replaces telnet and rlogin), scp (which replaces rcp and ftp), sftp (Secure FTP, which also replaces ftp), sftp-server, sshd (server-side), etc. Since the program is open source, providing strong authentication and secure sessions, it may presumably provide the base for your implementation of SSH. It is supports a wide number of operating systems: OpenBSD, NetBSD, FreeBSD, AIX, HP-UX, IRIX, Linux, NeXT, SCO, SNI/Reliant Unix, Solaris, Digital Unix/Tru64/OSF, and Mac OS X. In SSH1 mode, the encryption algorithms supported include 3DES and Blowfish. In SSH2, 3DES, Blowfish, CAST128, Arcfour, and AES are supported.
Network Security Services
Used in Netscape and Mozilla products, NSS (Network Security Services a.k.a Netscape Security Services) allows you to develop security-enabled server applications that support SSL v2 and v3, TLS, PKCS #5, PKCS #7, PKCS #8, PKCS #9, PKCS #10, PKCS #11, PKCS #12, S/MIME, X 509 v3 certificates, RSA, DSA, Triple DES, DES, RC2, RC4, SHA-1, MD2, MD5, etc. The software is released under the Mozilla Public License and the GNU General Public License.
LCRZO Network Library (Netwib)
This library provides support for UDP/TCP clients and servers, sniff, spoof, packet creation, packet decoding, packet printing, address conversion, etc. Platforms supported include Linux, FreeBSD and Solaris. The library is apparently released under the GNU LGPL. It is no longer actively maintained.
Datareel Database C++ Library
This is a collection of C/C++ libraries that purportedly "simplifies complex time-consuming database, socket, multithreaded programming tasks". It supports the Windows and Unix platforms.
CYRUS SASL Library
To quote, "the Cyrus SASL library is a generic library for SASL authentication, as used in SMTP and IMAP protocols. It's used for authentication by the Cyrus IMAP server as well as the Sendmail 8.10 release." It also includes a Java SASL library. Note that the link above is to an FTP directory. Look for a file like "cyrus-sasl-1.5.20.tar.gz" (the numbers may change when newer versions are released) there.
GNU Common C++
GNU Common C++ is a C++ framework for developing portable threaded applications. It has abstractions for threads, sockets, XML parsing, serialization, config files, and so on, hiding the differences between platforms in its classes so that you can write code that is (hopefully) portable. It is released under the GNU GPL license.
OPP C++ Class Library
A C++ class library that has a number of features, including classes for retrieving files via HTTP, sending and receiving email, etc. It supports Unix systems. The C++ source code is available.
LibTCP++ TCP/IP Client/Server Class Library
A C++ TCP client/server class library that is supposed to simplify and speed development of TCP/IP client/server applications. The Linux platform is supported. The C++ source code is available.
Copying files via FTP, HTTP, Gopher using Wininet.dll (WDJ Nov 96)
With this library your Windows program can have the capability to get files from the internet using FTP, HTTP and Gopher. Note that this tool relies on Microsoft's wininet.dll which part of Internet Explorer. The actual source code is contained in one of the ZIP files in the archive downloaded (each ZIP file has a README.TXT file that explains its contents).
Inettransferlib (HTTP and FTP Library)
InetTransferLib is a free library for Access 97 for obtaining documents via HTTP and FTP.
Apache Web Server
Source code for the most widely used Apache web server is freely available. It can be compiled and used on Unix systems as well as Windows (although at the time of this writing, the Windows code is not as stable as the Unix one).