I came across this rather late, and was surprised to see that nobody had mentioned calendar(1), which is practically standard on Linux and BSD systems, just like cal(1). It is quite useful for both reminders and different kinds of calendars. There are several standard calendars included, e.g. orthodox dates, jewish dates, birthdays of celebrities, etc. It is also capable of handling email from a cron job, and easy to configure.
But of course, this is a command line application, so it may not be for everyone. Still, even my brother, who is a Mac OS X die hard fan, uses it from time to time.
He. It takes up only 61 kibibytes. Good old Unix :-)