VDB SOCKET LIBRARY
VDB Socket Library is a C++ class library that you can use to build portable TCP/IP and RS232 data communication applications. Versions for Unix and Win32 are interoperable.

MIMELITE
This C library supports the subset of MIME that is required for the encoding and transfer of European character sets. Specifically, it supports the Content-Transfer-Encodings 7BIT, 8BIT, BASE64, and QUOTED-PRINTABLE. It also handles the conversion between the ISO Latin 1 character set and PC based character sets (eg CP-850, CP-437 and Macintosh). It was designed originally to work on MSDOS, where resources are scarce, but is apparently general enough for other contexts. The C source code is provided.

WS_FTP
If you use Windows, you'd probably have heard of WS_FTP. The link points to the complete source code for an early version of this famous ftp client.

CYRUS SASL
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.

MIME++
This C++ class library allows you to parse, create and edit messages in the MIME format. Their website notes that the library has a proven track record from field testing and is perfect for developing email, Usenet or web applications. It handles the quote-printable, base64, uuencode/uudecode, Binhex 4.0, MacMIME (applesingle and appledouble) encodings. It also supports SMTP, POP and NNTP protocols, multipart messages, messages with attachments. You can customize the class or extend it through inheritance and polymorphism. Price? Free for non-commercial, educational and personal use. It supports Win32 (Windows 95, 98 and NT), Linux, SGI Irix, Digital Unix, AIX, HP/UX, Solaris, and SunOS. If your application needs email capabilities, you can't afford not to check this out! Source code is available.

POP3 CLIENT
Get the files POP3.ASC and POP3.ZIP from that page to obtain a C++ POP3 client that uses Winsock. The package is distributed in the form of C++ source code.

APE
APE is a C++ framework for creating threaded network and telephony servers under Unix and Win32. It has abstractions for threads, sockets and synchronization objects. It is released under the GPL license (and as such comes with the C++ source code) with exceptions.

OPP
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++
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.

SMTP SendMail() (WDJ June 98)
Add email capabilities to your application by simply including the source code to this function in your application and calling the SendMail() function. This function sends email using the SMTP protocol (ie, it does not rely on MAPI being installed). This code is intended for the Windows platform. 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).

HTTP Client (WDJ Oct 99)
Add the ability to get web pages to your program. The source code here is short, so it is easy to read and understand, and will help you get started in understanding how to code for Winsock. Note though that the code is actually too simplistic and is not able to handle situations where the program has to use a proxy to access the outside world. 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).

Scanning TCP/IP Ports (WDJ Feb 98)
Source code for a utility program that will scan the TCP/IP ports under Windows 95 and NT. 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).

HTTP Client with MFC (WDJ Nov 97)
This is an MFC class for multithreaded HTTP downloads. Note that it requires your users to have the WININET.DLL installed. If they do not, you probably have to tell them to install Internet Explorer. Yeah, I know... I guess it depends on your user base. 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).

SendEmail() using MAPI (WDJ Nov 97)
Give your application email capabilities via MAPI (implying of course, that this package only applies to the Windows platform). Just call SendEmail() in your application with the addressee's email address, the subject and the text of your message and you're done. 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).

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. Alas, this tool relies on Microsoft's wininet.dll which is not necessarily present in every Windows platform. It is installed with Internet Explorer though, so if your users do not have it, you may have to persuade them to make a 30MB download (for IE) before they can use your program! 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).

MOZILLA
If you've not heard of this, you must be truly new on the scene or you've been on a really long vacation. This is the source code for Netscape (yup), released free of charge under an open source license. Writing an application that involves the HTTP protocol? Well, what are you waiting for?

WAR FTP DAEMON V2 ALPHA
The source code for a ftp daemon that can only be used in freeware programs (but not if you are the government, military or any political organisation). You have to fill in a form and the author will inform you where to get the code.

INETTRANSFERLIB
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).

STARDUST
If you are writing using winsock on Windows, you should not miss this site. It contains loads of information on programming for winsock and sample source code for a variety of programs as well (both Winsock 1 and 2). You can even get the source code for a TCP/IP stack here!