The Darwin Calendar Server is a standards-compliant server that allows multiple users to collaboratively share calendaring information. It provides a shared location on the network to store schedules, and allows users to send each other and manage invitations. In order to provide interoperability with multiple calendaring clients, the server implements the CalDAV protocol, which is an extension of WebDAV, which is in turn an extension of HTTP.
mod_athena is an Apache-based application load balancer for large systems. It allows the HTTP server to act as a load balancer either internally to Apache's own mod_proxy (for reverse proxying), or externally to machines querying it. Arbitrary statistics are sent to the engine via a simple GET plus query-string interface, from which it will then make decisions based on chosen algorithms.
Google Sitemap Generator is a tool installed on your Web server to generate sitemaps automatically. Unlike many other third-party sitemap generation tools, Google Sitemap Generator takes a different approach: it will monitor your Web server traffic and detect updates to your Web site automatically.
libapache2-mod-scramble-ip is an Apache 2 module that works like mod_removeip, but instead of just overwriting the IP address with 127.0.0.1, it encrypts the IP address. This way you always get an IP address to work with (in scripts, etc.) and have the ability to use tools like awstats to analyze your logs. It's in alpha status, but working on some Apache 2 servers, and the 'cost' (load) should be small and reasonable.