Modules

Apache Module Development in C
Companion site of a book written by Ken Coar for writing Apache modules.
http://apache-modules.com/

Apache Module Registry
Resources for module developers and users.
http://modules.apache.org/

Apache.org: Module mod_rewrite URL Rewriting Engine
Rewrites the requested URL on-the-fly based on configuration directives and rules.
http://httpd.apache.org/docs/mod/mod_rewrite.html

auth_ldap
Provides LDAP authentication.
http://www.rudedog.org/auth_ldap/

Chili!Soft ASP [Commercial]
Active Server Pages module for Apache.
http://www.chilisoft.com/chiliasp/

DSSI - SSI Extension
Dynamic function call extensions to mod_include
http://apache_dssi.tripod.com/

FrontPage
Microsoft FrontPage for Apache and other Web Servers.
http://www.rtr.com/fpsupport/

Getting Started with Java Servlets using Apache JServ
Java solution Guide for the Apache mod_jserv module.
http://www.devshed.com/c/a/Java/Getting-Started-with-Java-Servlets-using-Apache-JServ/

Java Apache Project
Java for the Apache Web Server. This archive is maintained purely for historic purposes. The Java Apache Project ceased its development in 1999 and all codebases were moved or superseeded by those found at the Apache Jakarta Project or Apache XML Project.
http://archive.apache.org/dist/java/

mod_auth_kerb
Provides Kerberos user authentication.
http://modauthkerb.sourceforge.net

mod_bandwidth
enable the setting of server-wide or per connection bandwidth limits, based on the directory, size of files and remote IP/domain.
http://www.cohprog.com/mod_bandwidth.html

mod_cntr
Counts and displays page hits.
http://www.dan.co.jp/webcounter/

mod_dav
Apache module to provide DAV capabilities (RFC 2518) for your Apache web server. It is an Open Source module, provided under an Apache-style license.
http://www.webdav.org/mod_dav/

mod_gunzip
Lets Apache decompress gzipped files before serving them to browsers that cannot.
http://www.innerjoin.org/apache-compression/howto.html

mod_gzip
speeds up load-time of Web sites by compressing HTML pages on-the-fly.
http://www.schroepl.net/projekte/mod_gzip/

Mod_plsql
Module for connecting Oracle databases to Web in a way. Compatible with Oracle Application Server. [Open source]
http://www.selfsoft.com/progs/mod_plsql/

mod_rewrite: A Beginners Guide
Covers the basics necessary for rewriting, through the use of examples. Includes how to see if the Apache module is installed, rule structure and how conditions are used.
http://www.workingwith.me.uk/articles/scripting/mod_rewrite/

mod_rewrite Reference
Quick reference to mod_rewrite's configuration directives.
http://www.apacheref.com/ref/mod_rewrite.html

mod_rewrite: URL Rewriting Guide
A guide to this powerful but complicated module.
http://www.engelschall.com/pw/apache/rewriteguide/

mod_ruby
Embeds the Ruby interpreter into the Apache web server, allowing Ruby CGI scripts to be executed natively.
http://www.modruby.net/

mod_security
Open source intrusion detection and prevention engine for Apache.
http://www.modsecurity.org/

mod_ssl
Apache Interface to OpenSSL
http://www.modssl.org/

Nona's mod_auth_ldap
Apache module for authenticating users against LDAP directories.
http://nona.net/software/ldap/

Pluggable authentication for Apache
Features mod_auth_pam, a module for pluggable authentication (PAM).
http://pam.sourceforge.net/mod_auth_pam/

RexxTags
A Rexx Server Pages (RSP) compiler, suitable for rapid prototyping of XML tags in REXX and for deployment in small-to-medium sized web sites. Requires mod_rexx.
http://www.rexxtags.org/

Robotcop
Helps webmasters prevent spiders from accessing the parts of sites they have marked off limits using the robots.txt file.
http://www.robotcop.org/

URL Rewriting
Beginner's guide to using Apache's mod_rewrite. Includes why redirects are necessary, basic use, and regular expressions.
http://www.yourhtmlsource.com/sitemanagement/urlrewriting.html