Apache Reference:

Core Functionality
http_core Apache Base Functionality
mod_so Dynamic Shared Object (DSO) Bootstrapping

URL Mapping
mod_alias Simple URL Translation and Redirection
mod_rewrite Advanced URL Translation And Redirection
mod_userdir URL Selection by User Names
mod_imap URL Selection by Image Map Coordinations
mod_speling URL Spelling Correction

Access Control
mod_access Host- and Network-Based Access Control

User Authentication
mod_auth User Authentication by User Name/Password
mod_auth_anon User Authentication by Anonymous Name/E-Mail Address
mod_auth_dbm User Authentication by User Name/Password (UNIX NDBM)
mod_auth_db User Authentication by User Name/Password (Berkeley-DB)
mod_digest User Authentication by User Name/Realm/Password

Content Selection
mod_dir Content Selection by Using Directory Default Documents
mod_actions Content Selection by Content Types and Request Methods
mod_negotiation Content Selection by Best-Matching Client Capabilities

Environment Creation
mod_env Fixed Environment Variable Creation
mod_setenvif Conditional Environment Variable Creation
mod_unique_id Generation of Unique Identifiers by Request

Server-Side Scripting
mod_cgi Common Gateway Interface (CGI) Implementation
mod_include Server-Side Includes (SSI) Implementation

Response Header Generation
mod_mime Fixed Content Type/Encoding Assignment
mod_mime_magic Automatic Content Type/Encoding Assignment
mod_expires Creation of HTTP Expires Header
mod_headers Creation of Arbitrary HTTP Headers
mod_cern_meta Creation of Arbitrary HTTP Headers (CERN-style)

Internal Content Handlers
mod_asis Generation of Raw Responses
mod_autoindex Generation of Directory Index Documents
mod_status Display Summary of Server Runtime Information
mod_info Display Summary of Server Configuration-Time Information

Request Logging
mod_log_config Generic Request Logging
mod_log_agent Specialized User-Agent Logging (Deprecated)
mod_log_referer Specialized Referrer Logging (Deprecated)
mod_usertrack Specialized User Click-Trail Logging

Experimental
mod_mmap_static Caching of Frequently Served Pages via Memory Mapping
mod_example Apache API Demonstration (Developers Only)

Extensional Functionality
mod_proxy Caching Proxy Implementation for HTTP and FTP
mod_perl Perl Integration and Interface
mod_ssl SSL/TLS Integration and Interface