Openfire (formerly Wildfire) is a cross-platform real-time collaboration server based on the XMPP (Jabber) protocol.

OpenIM Java Jabber ® Server is an open-source Java implementation (BSD License) of Jabber Instant Messager.
The purpose of the OpenIM project is to produce a fast, simple, and highly efficient instant messager server with high modularisation codebase.

shicks! - a portable, free POP3/SMTP server.
Shicks! has a BSD-style license. Earlier versions were GPL-licensed, but that is too restrictive for some uses.

hMailServer is a free, open source, GPL-licensed e-mail server for Microsoft Windows. It supports all the common e-mail protocols (IMAP, SMTP and POP3) and comes with an easy-to-use COM library that can be used for integration with other software. It also has support for virtual domains, distribution lists, antivirus, antispam, aliases, distributed domains and much more. E-mail data is stored in a database server, MySQL or MS SQL, depending on your choice.

The Apache HTTP Server Project is an effort to develop and maintain an open-source HTTP server for modern operating systems including UNIX and Windows NT. The goal of this project is to provide a secure, efficient and extensible server that provides HTTP services in sync with the current HTTP standards.

Xitami :: small, fast, and powerful multiplatform Open Source web server.

Savant is a freeware open source web server that runs on Windows 9x, ME, NT, 2000, and XP turning any desktop computer into a powerful web server.  Designed to be fast, secure, and efficient, Savant is the choice of thousands of professional and amateur webmasters worldwide.

MyServer is a powerful and easy to configure web server. Its multi-threaded architecture makes it extremely scalable and usable in large scale sites as well as in small networks, it has a lot of built-in features. Share your files in minutes!

Jigsaw is W3C's leading-edge Web server platform, providing a sample HTTP 1.1 implementation and a variety of other features on top of an advanced architecture implemented in Java. The W3C Jigsaw Activity statement explains the motivation and future plans in more detail. Jigsaw is an W3C Open Source Project, started May 1996.

OpenNap :: Napster is a protocol for sharing files between users. With Napster, the files stay on the client machine, never passing through the server. The server provides the ability to search for particular files and initiate a direct transfer between the clients. In addition, chat forums similar to IRC are available. OpenNap extends the Napster protocol to allow sharing of any media type, and the ability to link servers together.