mod_ntlm is a NTLM authentication module for Apache / Unix. NTLM is often used in Microsoft based Intranets to pass user's credentials from their workstation-logon to the web server (user authentication without password boxes). Password validation is done through Windows NT or Samba machines over TCP/IP.
ctm.pl is a Perl script which generates Apache type maps for use with mod_negotiation. Useful if you have a lot of images available in different formats you need type maps for, especially if you don't want to use MultiViews to decide which of them will be sent back to the requesting browser.
FilterProxy is a Perl script that acts as a generic Web proxy. It is unique in that it allows you to install modules that can perform arbitrary transformations on HTML (or any other MIME-type) and HTTP headers. It filters ads by stripping HTML from the page, anonymizes requests by removing Referer and User-Agent headers, compresses HTML content, and de-animates animated gifs. Configuration is done via Web-based forms or editing a Perl data structure.
mod_repository turns Apache into a content storage system with the benefit of storing just one copy of each file submitted to the repository. Upon storing a file a user is given a unique token representing the file. This can be used to create content servers which are abstracted from the filesystem. Files can be placed, retrieved, and deleted from the server.
mod_proxy_add_forward.c is an Apache module that adds a 'X-Forwarded-For' header to outgoing proxy requests like Squid does. You can then get the client IP back on the proxied host by setting r- >connection->remote_ip from this header. It's very useful for the common lightweight frontend/heavy backend mod_perl setup.
mod_atrack adds advanced user tracking functionality to the Apache Web server. It was based on mod_usertrack and introduces many new configuration features primarily designed to reduce the annoyance of sending multiple cookies to a single user and allow more intelligent analysis of collected data.