Diary
Simple journal keeping, with output in various formats, including HTML.

Stephen Murphy's Software - MakeCal
Generates wall planners/calendars in Draw format

Explan Diary Software
Formerly commercial products, the Event diary and two types of the networked Occasion diary, are now FreeWare.

Chris Morison's Risc Organizer
Desktop personal Organizer for Acorn RISC OS Computers. It contains all the main features of an Organizer - calendar, diary, anniversary and address-book sections within a simple, user friendly graphical interface.

RemindMe
For people working in a busy environment, where major tasks can be easily overlooked in favour of minor ones.