For you GMail users (if you aren’t one, get an account immediately!)
take note of the new calendar feature. It has tight integration with
your GMail inbox so you can create an event from an email. You can, of
course, create events outside of your inbox and have the calendar send
you reminders ahead of time (hint: birthday’s and anniversary’s).

It’s a really nifty addition, and you can’t beat the price of free.