This is the list of the standard apache2 modules that are installed by the Debian apache2 package: _actions, asis, auth_anon, auth_dbm, auth_digest, auth_ldap, cache, cern_meta, __cgi, _cgid, dav, dav_fs, deflate, disk_cache, expires, ext_filter, file_cache, headers, imap, include, info, ldap, mem_cache, mime_magic, proxy, proxy_connect, proxy_ftp, proxy_http, rewrite, speling, ssl, suexec, unique_id, userdir, usertrack, vhost_alias.
Here is a short description and the link to the apache docs for each of the modules from above:
mod_actions: This module provides for executing CGI scripts based on media type or request method.
mod_asis: Sends files that contain their own HTTP headers
mod_auth_anon: Allows “anonymous” user access to authenticated areas
mod_auth_dbm: Provides for user authentication using DBM files
mod_auth_digest: User authentication using MD5 Digest Authentication.
mod_auth_ldap: Allows an LDAP directory to be used to store the database for HTTP Basic authentication.
mod_cache: Content cache keyed to URIs.
mod_cern_meta: CERN httpd metafile semantics
mod_cgi: Execution of CGI scripts
mod_cgid: Execution of CGI scripts using an external CGI daemon
mod_dav: Distributed Authoring and Versioning (WebDAV) functionality
mod_dav_fs: filesystem provider for mod_dav
mod_deflate: Compress content before it is delivered to the client
mod_disk_cache: Content cache storage manager keyed to URIs
mod_expires: Generation of Expires and Cache-Control HTTP headers according to user-specified criteria
mod_ext_filter: Pass the response body through an external program before delivery to the client
mod_file_cache: Caches a static list of files in memory
mod_headers: Customization of HTTP request and response headers
mod_imap: Server-side imagemap processing
mod_include: Server-parsed html documents (Server Side Includes)
mod_info: Provides a comprehensive overview of the server configuration
mod_ldap: LDAP connection pooling and result caching services for use by other LDAP modules
mod_mem_cache: Content cache keyed to URIs
mod_mime_magic: Determines the MIME type of a file by looking at a few bytes of its contents
mod_proxy: HTTP/1.1 proxy/gateway server
mod_proxy_connect: mod_proxy extension for CONNECT request handling
mod_proxy_ftp: FTP support module for mod_proxy
mod_proxy_http: HTTP support module for mod_proxy
mod_rewrite: Provides a rule-based rewriting engine to rewrite requested URLs on the fly
mod_speling: Attempts to correct mistaken URLs that users might have entered by ignoring capitalization and by allowing up to one misspelling
mod_ssl: Strong cryptography using the Secure Sockets Layer (SSL) and Transport Layer Security (TLS) protocols
mod_suexec: Allows CGI scripts to run as a specified user and Group
mod_unique_id: Provides an environment variable with a unique identifier for each request
mod_userdir: User-specific directories
mod_usertrack: Clickstream logging of user activity on a site
mod_vhost_alias: Provides for dynamically configured mass virtual hosting