Celebrat is a very easy-to-use, non-interactive, text-mode calendar application. It reads a data file in ASCII format, and prints on stdout a human-language summary of what events will take place up to ten days from now. It also includes a small daemon which announces events that are due by putting a message on every registered terminal, or by integrating the announcement in the bottom line of GNU screen sessions.
Citadel is an advanced messaging and collaboration system for groupware and BBS applications. Users can connect to Citadel using any telnet, WWW, or client software. Among the features supported are public and private message bases (rooms), electronic mail, real-time chat, paging, shared calendaring, address books, mailing lists, and more. Unlike other collaboration servers, Citadel provides its own data stores and is therefore extremely easy to install; you don't have to "bring your own" email and database because they're built in. The server is multithreaded and scalable. In addition, SMTP, IMAP, and POP3 servers are built-in for easy connection to Internet mail. Citadel is both robust and mature; it has been in production since 1987.