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