,----[ Quote ]
| Kapor estimates there are as many as a few thousand active Agenda
| users, although he personally has stopped using the program because of
| its age and lack of updates. Currently, in addition to his open source
| calendar, he uses text documents with programmed-in macros to
| track information -- and he eagerly awaits the first full release
| of Chandler.
An alpha is available (for Linux too):
There's also Sunbird:
Or the calendar extension for Thunderbird, among many others.