Project-Based Calendaring System (PBCS) is an open source web-based calendaring system, designed to be a useful tool for project managers or project planners. In PBCS you can have appointments of multiple people in one overview, making it easy to plan appointments for projects in the future. Of course the people whose appointments are being scheduled have special overviews to see them all at once for a week or a month. Next to appointments you can add notes as reminders of things to do.
I have pointed out many weblogging tools/content management systems in the past on this blog. I think calendaring is fairly on topic, and this seems to look like a pretty good one, though I have never seen the perfect calendaring system yet.