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_psldap is an Apache module that performs authentication and authorization against an LDAP server with LDAP based session management. It also provides Web 2.0 based capabilities to add, edit, move, and create new records in the LDAP store, leveraging XSL stylesheets to offload heavy processing to the clients and reduce bandwidth consumption by up to 95% or more.
mod_replace is an Apache 2.0.x filter module that allows you to replace text strings based on regular expressions. It is most commonly used together with mod_proxy to sanitize the behaviour of ill-behaving Web applications and servers when using Apache as an reverse proxy. Other uses are, of course, also supported.
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_reserve is an Apache2 module for Linux that extends the Apache HTTP server, allowing it to reserve a certain CPU bandwidth for the provisioning of a service. This module exploits the functionality provided by AQuoSA, whose API is used to reserve CPU bandwidth for execution of tasks. In this way, the module can force the Web server to provide services, like CGI or Perl scripts, using exactly the configured CPU bandwidth. This behaviour is useful in order to provide Quality of Service to certain types of services, or to prevent the kinds of attacks in which a service can starve all the others.
With mod_roaming you can use your Apache webserver as a Netscape Roaming Access server. This allows you to store you Netscape Communicator 4.5 preferences, bookmarks, address books, cookies etc. on the server so that you can use (and update) the same settings from any Netscape Communicator 4.5 that can access the server.
mod_sar ("sar" stands for Search And Replace) is an Apache2 module that works as an output filter. Its purpose is to search for and replace strings found in Web content before sending the content to the client. Searches performed can be case sensitive or case insensitive, depending on configuration. A good example of common usage of this module is a reverse proxy.