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.
Mod_auth_kerb is an Apache module designed to provide Kerberos user authentication to the Apache web server. Using the Basic Auth mechanism, it retrieves a username/password pair from the browser and checks them against a Kerberos server as set up by your particular organization. It also supports mutual ticket authentication, although most browsers do not support that natively.
mod_backhand is a load balancing module for Apache. It provides per-request HTTP redirection within a heterogeneous Apache server cluster. Each request is processed and run through a set of "candidacy functions" to determine which server is best suited to respond. The request is then proxied to that server. Facilities are in place to allow you to write your own dynamically loadable decision making algorithms. Everything about the request and the current availability of resources can be used in the decision-making process.
mod_dav enables Apache to understand the WebDAV protocol (extensions to HTTP). WebDAV stands for "Web-based Distributed Authoring and Versioning", and is defined by RFC 2518. WebDAV is intended to replace proprietary authoring protocols such as those used by Frontpage or NetObjects' Fusion, but is also a complete set of protocols for manipulating a webserver's files and directories, and their properties.