Servers

ASPSeek
An Internet search engine, consisting of an indexing robot, a search daemon, and a CGI search frontend. (C++) [GNU/Linux, FreeBSD]
http://www.aspseek.org/

Binc IMAP
Open source xinetd/tcpserver based IMAP server for the Linux platform, with support for Maildir.
http://www.bincimap.org/

Bind
Widely-used DNS server software, and related tools. (C) [Many platforms]
http://www.isc.org/sw/bind/

Board-tnk
Discussion board with admin page in English, French, German, Italian, Spanish, and Dutch. (PHP4)
http://www.linux-sottises.net/software.php

B.Y.P. Server
A server for applications running into thin-clients PCs. It resembles the Web in many ways, but supports stateful sessions. (Python)
http://members.fortunecity.it/alessandro70/

Courier mail server
An SMTP/IMAP/POP3/HTTP (webmail) server, implementing SMTP extensions for mailing list management and spam filtering. (C, C++, Perl) [GNU/Linux, Unix]
http://www.courier-mta.org/

Cyrus IMAP Server
An IMAP server developed at Carnegie Mellon University. (C, Perl) [GNU/Linux, Unix]
http://cyrusimap.web.cmu.edu//

DataparkSearch Engine Tool
Open source search engine tool released under GPL and designed to organize search within a website, group of websites, intranet or local system.
http://www.dataparksearch.org/

dnsadmin
A web control panel for dns administrators. DNS information is stored in mysql and can be output to either BIND or tinydns files. (C) [GNU/Linux, Unix]
http://www.inter7.com/index.php?page=dnsadmin

Dnsupdate
An IP address update program for dynamic DNS updates as defined in RFC2136. It submits dynamic IP address update requests to a name server automatically. (Perl) (Linux)
http://www.btree.org/dnsupdate/

Domain Technologie Control
A set of PHP scripts for domain administration, MySQL management database, backup scripts generation and scripts for statistic calculation.
http://www.gplhost.com/software-dtc.html

Exim - Mail Transfer Agent
Message transfer agent (MTA) developed at the University of Cambridge for use on Unix systems connected to the Internet.
http://www.exim.org/

Ganymede
A system for managing a variety of directory services, such as NIS, DNS, Samba, Sendmail, and tacacs+. (Java)
http://tools.arlut.utexas.edu/gash2/

ht://Dig
A complete world wide web indexing and searching system for a small domain or intranet. (C++) [GNU/Linux, Unix]
http://www.htdig.org/

HTML::Embperl
Gives you the power to embed Perl code in your HTML documents and the ability to build your Web site out of small reusable objects in a OO-style. (Perl)
http://perl.apache.org/embperl/

ihttpd
A tiny and simple http server that runs under inetd. It supports HEAD/GET/POST, and IPv6.
http://i1.dk/download/ihttpd/index.html

IRC Services
Provides IRC networks with definitive nickname and channel ownership, allows messages to be sent to offline users, and gives IRC operators more control. [GNU/Linux, Unix]
http://www.ircservices.za.net/

MnoGoSearch
Search engine software for Internet and Intranet sites. The index can be stored in any of the popular SQL Databases.
http://mnogosearch.org/

MyServer
Web server administration supporting HTTP, CGI, ISAPI, WinCGI and FastCGI protocols.
http://www.myserverproject.net/

nginx
A HTTP and mail proxy server licensed under a 2-clause BSD-like license. By Igor Sysoev. [Unix, Mac OS X]
http://www.nginx.net/

OpenDS
A community project building a free and comprehensive directory service, based on LDAP and DSML standards. (Java)
http://www.opends.org/

OpenProtect
Open source e-mail filtering solution for Linux e-mail servers. (C, Perl) [Linux, BSD]
http://www.openprotect.com/

Postfix
A compatible, more secure replacement for sendmail program, which transfers mail over the internet. (C) [GNU/Linux, Unix]
http://www.postfix.org/

Teapop
An RFC1939 and RFC2449 compliant POP3-server, with virtual domain support and the possibility to authenticate users in several different ways. (C) [GNU/Linux, OpenBSD, Solaris]
http://www.toontown.org/teapop/

tlsproxyd
Daemon which proxies TLS connections into services which don't support TLS themselves. Readme file, change log, man page, and tarballed source. Requires OpenSSL. (C) [GNU/Linux, Unix]
http://www.ex-parrot.com/~chris/tlsproxyd/

tpop3d
Pop 3 server designed to be fast, extensible, and secure. Version history, FAQ, overview, and tarballed source distribution. (C) [GNU/Linux, Unix]
http://www.ex-parrot.com/~chris/tpop3d/

UnrealIRCd
An IRC daemon with SSL support and WebTV support. (C) [GNU/Linux, Unix, Windows, Amiga]
http://unrealircd.com/