WebCal is a free browser-based calendar program to help you keep track of your appointments, meetings, birthdays, etc. It's simple, yet has powerful scheduling features, email reminders, and a user-friendly interface, and being Web-based makes it ideal for use by groups of people.
E*Reminders is a series of PHP scripts that interact with a MySQL database to setup email reminders for any events or occasions you'd like. It includes recurring reminders, advance notices, and password authentication for email addresses. It can be used for an entire company, as a service for Internet users on your site, or privately.
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.
WebTTS is a WWW-based trouble ticketing system designed for businesses that need to track problems in their network, schedule system maintence, and track general customer problems. Most large businesses use a tracking system to help deal with customer problems, but these systems are either non-free, custom written, or simply impossible to install and get working in a reasonable amount of time.
XNotesNG is the next generation of XNotesPlus, a full-featured personal information manager. It is plugin driven and includes a Todo Manager, Calendar Manager, Category Manager, and extensive notes system, including features such as text searching, printing, alarms, date and calendar inserts, and categorization by colored projects.
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.