CalendarTechnique is a Web-based calendaring and scheduling application. It lets you easily post, maintain, update, and archive calendar and scheduling entries such as events, tasks, and journal entries. It features displaying calendar entries in customizable day, week, month, year, or list layouts, extensive search and filtering capability (keyword search, field-specific search, date range search), support for the event, to-do/task, journal, and alarm components specified in the iCalendar specification, and a theme-based interface which allows you to preview and select any of the available themes (new themes can also be added and customized to match the look of an existing Web site).
GetMeDone is a task manager in the spirit of Getting Things Done. It uses PostgreSQL as a backend and is written in Tcl/Tk. Currently only tested on Linux, it should be easy to extend to run on Mac OS and Windows. It features easy filtering of contexts, priorities, and time, advanced recurrent tasks, linking tasks to projects, deadlines, an ideas list, and the ability to defer tasks to future dates.
Zadolbator is a simple reminder component for Jabber server. As it is installed on the server side, it can be used by all users of this Jabber server, and does not depend on the client software used. Its main purpose is to remind you about events such as birthdays or holidays, though it can be used also for keeping your everyday reminders about meetings, things to do, etc. It uses the iCalendar format to store events, so some basic understanding of iCalendar is needed to use it. This also simplifies importing existing events if they're in iCalendar format.
TaskSheet is a hierarchical task management and timesheet application. It allows you to create a task along with all the tasks necessary to complete that task, and so on. for all eternity. This makes it easy to figure out what your "next action" should be and to determine which tasks took the longest.